Why speedrunners use NES instead of FDS? It's important beause I use FDS get a 5:05.963(allow RTA rules' PB)

I am trying speedrun with a 5:05,I get a 5:06 two days ago,and the hammer bros are so tricky,I lose 1 framerule 8-3 and slow down in 1-2 and 1 framerule slow 1-1,4-2 3 dumps to get a 353, 8-1 was 198, 8-2 is a hard level for me because it's to random,I try to control the emeny but it's too diificult I always lose time on stupid place, I want to do BBG to improve my grade,But bullet bill usually doesn't shoot So, guys how can I improve my grade,I need help.

English is too hard