Please tell me why Mesa put 90 watt speaker into Mesa Mark V and 200 watt speaker into Mesa Mark III. These amps have similar power so why is so big difference between speakers?
It's more about the sound and less about wattages.

The speakers they could source cheaply and still sound good changed, pretty much.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
^ agree probably just what they felt was right at the time. The speaker wattage most likely has nothing to do with it, both speakers can handle both amps.

I dont know what was in the Mark III but I absolutely hated the C90 in my Mark IV and quickly bought a 2x12 with V30s to run it through.
Yeah the early Mark IVs had it right with the EV speaker. I don't know what happened, if EV was just too expensive or if they wanted to cash in on Celestion popularity, but that switch was definitely the worst choice Mesa has made, barring the three-channel Recs apparently.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.
EVM is heavy as hell, great speaker though. I too prefer V30s for my Mesa shenanigans.
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY