ELI5: Why is it that abortions are so frowned upon and hard to get, yet selective reductions seem to be pushed for quite often?

I've read that even when there are significant health risks for the mother, it can still be quite a harrowing process to receive an abortion (mostly what I've found is from USA). However, in the case of multiple fetus pregnancies, the exact opposite seems to be the case.

Is it because women that have these pregnancies are often already undergoing fertility treatment in special clinics? The reasons for these reductions are all the same as getting a "standard" abortion. Mental/emotional/financial preparedness, health risks...

Why do these seem to be treated differently?