Short answer: power and speed creep.
Long answer:
>zone
still good, still OU
>lickilicky
was never good
>rhyperior
was always decent at best in OU, decent in UU
>tangrowth
actually got better over time, is ok in OU but trending down
>vire
was never good but team preview in gen 5 made everyone realize it
>magmortar
was never good
>PZ
was always niche, Z omniboost brought it back up for a while but kommo-o does the same thing better
>togekiss
always hovered around being niche in OU, isn't bad but there are things that break stall more quickly
>ambipom
was never good
>yanmega
no op, the bug/flying type introduced in the same gen as stealth rock was never more than bottom tier OU, you should not be shocked that it dropped
>honch
actually got better but it wasn't enough due to power and speed creep
>mismagius
was always niche at best in OU, we got better np users and better ghost types so that niche dried up
>gliscor
still OU, still good
>weavile
still OU, still good
>mamo
still solid in OU, not OU by usage because we have better ground types and you'd rather hit the 4x weak to ice things with a special ice move from something faster than them (greninja) than a weak physical ice shard
>gallade
was always niche in OU at best, be thankful for the mega
>probopass
was never good, and it was a magnet pull pokemon introduced at the same time as magnezone
>gatorade
was never great in OU, and as time went on its niche was filled by better spikers/tspikers
>duskinoir
lol
>frosslass
leads died out, even then there ended up being better and faster spikers like greninja. Was still somewhat viable in OU until SM.
You've also got to remember for things like electivire that all pokemon that are OU aren't necessarily good. Look at trevenant in XY. If enough retards use it, it'll become OU.
