Because all those other issues involve practical judgments, not moral choices.
TMC:
Do they? The Church teaches that they are all moral choices. And the judgment that the best way to reduce abortion is to make it illegal is surely a practical judgment, as is the same judgment for gay rights. The Church doesn’t teach anywhere that those two issues are the only ones that matter, or that all other issues are reduced to “practical judgments.” A Catholic can decide for himself that those two issues outweigh all other issues, but the Church says all the issues must be weighed. To say that only those issues matter, whether for voting or anything else, is simply not consistent with Catholic teaching.
No, the church nowhere teaches this. Nor does the fact that I can choose to act immorally mean that the problem I face is a moral issue rather than a practical one. If my car won’t start because my battery is dead, this is a practical problem, not a moral question even though one possible solution is to steal another battery. Immigration, health care, the budget etc are all alike in that determining the best solution is a practical concern, not a moral one, and the fact that immoral solutions can proposed doesn’t make the issues moral questions.