Why Apple Pay don’t availible in Latvia?
I’m asked same question to my bank (Swedbank, one of the biggest in Baltics) and get response that they ready to implement Apple Pay, so this festure doesn't get support from Apple side.
You would have to ask Apple why, we are fellow users (these are user forums) we won't know if/when they might add it.
If you want to leave feedback to Apple about it : http://www.apple.com/feedback/