Let's stick with criminal law here. One obvious reason why "immoral" doesn't entail "illegal" is that what's legal, what's not, and what the punishments are needs to be clear. In a functioning legal system, it's generally possible to determine in advance whether something is a crime, and in cases where it's not clear, there's a system for settling the matter, with various safeguards and forms of appeal built in. But there are plenty of moral loose ends — matters on which people disagree, sometimes vehemently, about whether something is immoral.

We might try restricting things by saying that actions which are clearly immoral should be illegal. Unfortunately, however, that doesn't move the ball as far as it would need to go. When people disagree vehemently about moral matters, one side typically thinks something is clearly immoral and the other side that it clearly isn't. Few of us would want to live in a state where we might be subject to imprisonment because some judge judges that something we think is moral is actually immoral. I may think that a statutory law is a bad one, but I can at least know in advance what the law actually is and work to get it changed if I want to.

In addition to these more general reasons, there's sheer practicality. Legal systems aren't meant to address all problems or govern all behavior, and thank Heavens for that. For one thing, the resources it would take to enforce all of morality through the law would be extraordinary, and better spent in other ways. But also, most people — I'm one — think that there should be a sphere of life that's outside the reach of the State. If morality is entirely swallowed up by the law, that sphere will shrink to almost nothing. And again: I suspect that very few of us would want to live in a country like that.