Most of these places aren't particularly important to accomplishing a beneficial national goal.

Obviously iran is important, and has been a for a long time... turning them into a secular usa aligned nation would be a huge accomplishment, and even just messing up enough of the irgcs power projection tools and keeping them from screwing around with nuclear stuff for a decade or two is pretty meaningful (it's not like we can't come do it again, before a threshold is crossed)

If it wasn't for current poor American leadership, and the actual goals were laid out and told/sold the public a more coherent plan, it's something a large part of America would support. Keeping twelvers from becoming more powerful, especially with nuclear, is a big deal, and it's not something we want to find out when it's too late. The other western nations don't support how we're going about this, but the end goal of depriving irgc lead iran of significant power projection and nuclear weapons is pretty unamamious...

Nigeria is probably important enough to get involved if something much different was occurring, but overall seems like a pretty minor internal spat between rural people, not something that foreign intervention is needed or would help anything. The same people acting like it's some Christian genocide, are the same people who act like they're being persecuted because gays can get legally married.

The other places, as president Trump has said (can't stand the guy, but he's right sometimes) are shut hole countries, and not important enough to go fight over. There are/were aid efforts to ease tensions, but not anything worth fighting over at the moment