Why is sodium the only allowed mod for 1.16.1? I have sodium+krypton+phoshpor+lithium and even then I'm getting constant fps drops because my computer is very bad. And why is only one specific version of sodium allowed?

Sodium is not the only allowed mod for 1.16.1. The mods listed as 1.15+ just above ARE allowed for 1.16.1 too. That's what the "+" means (as in 1.15 and every subsequent version). It's just that if you use sodium in 1.16.1, you have to use the special speedrun version of it.

Hope this helps :)