Call the hypothesis that God grants everybody efficacious grace 'EEG' for short. The first thing I'd say is that the above question presupposes something we cannot know: ceteris paribus, God would have good reason to give everybody efficacious grace. Now one might object at once that we surely can know there is such a reason: it would just be "better" for people if God precluded the possibility of serious sin. If EEG is possible, then that supposition seems reasonable enough. And it must be conceded that EEG is logically possible, in that it would be compatible with God's goodness and power, prescinding from what's actually been created and redeemed. But that doesn't establish that EEG is really possible. Countless things are logically possible for God which, all the same, are not possible given what he's already done. That's part of what motivated the old and crucial distinction between God's potentia absoluta and God's potentia ordinata. The former is a much broader category than the latter, and the mere fact that EEG falls within the former is no reason to believe that it also falls within the latter. So far, we have been given no reason to believe that EEG is possible given the general order of creation God has actually decreed—and that's aside from the question whether anybody's actually been given efficacious grace.
That "general order" of things is, I take it, the proximate reason why we all (with the few usual exceptions understood) inherit "original sin"—a state of alienation from God that only our first parents did a thing to bring about. Unless the general order of creation is such as to ensure the inheritance of original sin once the first sin was committed, there's no particular reason to believe that original sin is anything more than an arbitrary imposition on those who aren't responsible for it. Of course God is incomprehensible: we can't fully wrap our minds around God's essence or even his providence. But as the Pope argued well in his Regensburg address, that does not mean that any of God's actions are arbitrary. Even when God's "reasons" for doing certain things are opaque to us—which they often are—and even when they are not necessitating—in the sense that, if we knew them, they would not show that God had to do what he did rather than not—just their being God's reasons suffices to make his actions rational. For God is, among other things, Wisdom itself.
Now given that God has decreed a general order of things within which innocents inherit original sin, there's no reason to believe that EEG is possible without violating that order. Why? Well, as a matter of fact, God doesn't give efficacious grace to most people. If that choice is not arbitrary, that's almost certainly because everybody's getting it would undermine the integrity of said order, which would be tantamount to overthrowing it. At any rate, I can't think of any other reason, and I would never say as a Catholic that God arbitrarily decrees that some people are guaranteed heaven while most are not. It's more reasonable to suppose that he has his reasons for giving efficacious grace only to a few. Hence if, as is generally understood, God's decreeing the general order of things is eternal, then it's not possible for him to do anything to undermine that order's integrity. This is why, in general, miracles must be rare—at least relative to the sum total of events. But God can still do occasionally what he cannot do in the general course—such as grant efficacious grace.
It won't do to object that my defense is idle because the putative "general order of things" already entails the falsity of EEG. For the original question has force only on the supposition that EEG is possible given the rest of said general order—i.e. that EEG is really possible, not just logically possible. Only then could it be argued that it would have been "better" for God to have given everybody efficacious grace, ceteris paribus. But my argument is, precisely, that there is no reason to adopt the original supposition.
Even so, there is another objection worth considering. It can be posed by reframing the original question. The question is no longer, simply, why God doesn't grant efficacious to all if he grants it to some; the question rather, becomes why God decreed that order of things in the first place, when he could have decreed a different one in which EEG would have been the case, or at least possible. Although that question cannot be answered with doctrinal certainty, it is at least a fair question.
My answer, I believe, is traditional: God has good reason for decreeing an ordo salutis characterized by infinite, radical mercy rather than one in which that degree of mercy would be unneeded. An order in which nobody seriously sinned would be one in which either nobody is called to theosis at all or nobody is allowed to lose the divine life to which we are called. Theosis, assuming that's our vocation, would be a given and a guarantee, not a process which can fail by human choice. But the Incarnation, Passion, and Resurrection of the Son indicate that God had reason to manifest his love for us as infinite, radical mercy. The Passion would not have occurred if serious sin had not, and would not have manifested the degree of mercy it does if serious sin had not been virtually ubiquitous. The virtual ubiquity of such sin is incompatible with EEG. Yet it is not only compatible with, but apparently necessary for, the degree of mercy and love that God actually shows us. And that very degree of mercy is reason enough for Love itself to show it.
At this point, the only objection I hear is one that I've heard before: EEG is false because efficacious grace is per se incompatible with human freedom. Whatever would preclude human freedom is a fortiori incompatible with a free response to divine grace, which is what God beckons each of us to make. Hence there can be no efficacious grace for anybody, not just for everybody.
But there is a fairly obvious response to that objection. It is the belief of East and West that baptized infants who die before becoming psychically capable of serious sin go to "heaven," i.e. live forever in a state of blessed union with God. For them, theosis is a given not a process—at least not in this life. Is any Christian theologian prepared to say that such fortunate souls are mere automata who, as such, cannot love God? Of course not. If, as the present objection rightly assumes, love requires freedom, then such souls are free even though they've never had any opportunity to commit actual sin. By the same token, all the blessed in heaven are unaable to sin. Accordingly, there's no reason to suppose that some small minority of adults on earth can gain the privilege of incapacity to sin only at the cost of their own freedom or of God's justice to the rest of us.