In 1990 Warren lead the league in passer rating, TDs and yardage. He lead the oilers to a 9-7 record and the playoffs. But it was Joe Montana who won the MVP because his 49ers were 14-1. Montana was 2nd in TDs with 26, 4th in QB rating and 4th in passing yards.
The Associated Press mentioned that montan should win the award because his teams record and his stats. This is also the year Cunningham threw for 30 TDs and had almost 1000 yards rushing (though the maxwell club did award Cunningham the MVP) the knock against moon not getting the MVP was because his team was only 9-7 and his team ran the run and shoot so he should be penalized because his team passed so much. Thus moons stats were inflated.
Fast forward to 1998. Cunningham playing with the Vikings throws for 30 TDs and leads the Vikings to the best record in the league at 15-1. He does not get the MVP because Terrell Davis gets it since he rushed for 2k yards and lead his team to a 12-4 record.
The Associated Press feeling was that a running back who leads there team to the playoffs and gets 2000k yards should win the MVP. But we all know that was not the case with Eric dikkerson in 1984 (year Marino threw all those TDs) and in 2003 when Jamal Lewis ran for 2000k yards with taking that Ravens team with trash QB play to the playoffs that year.
Also that year the associated press gave the award to manning and McNair was only because Mcnair had some superman type of games in which he got out the hospital bed on a Sunday or Saturday and played. He had mutiple issues that year with cracked ribs a damaged sternum and a dislocated thump on his throwing hand. He also lead the league in QB rating. This was also a year where Eddie George began to show signs he was going down hill and had been ran into the ground. So folks thought the Titans were gonna take a step back but fisher idiot ass finally open the playbook up and let McNair throw. That year there was a game where McNair matched Peyton in a shoutout. Folks in the media were surprised because the Titans had no weapons yet were standing toe to toe with the colts who had given manning number 1 picks all at the skilled positions.
But IMO if he did not have those games where he got out of the hospital bed and played the same day he would not have won the MVP cause the AP has always looked for reasons not to give the MVP award to black QBs.
Edit- side note the year first year culpepper was a starter he lead the Viking to the second best record in the NFC and threw for 33 TDs (ran for 7 TDs) the most in the league. However there was push back on culpepper getting the award because he was a first year starter and the view was that he was on a ready made team and his stats were inflated because of Moss and he had Robert smith. The award was eventually given to Faulk.
Aside from Moon in 1990, none of these other QBs you mentioned deserved the MVP award those years.
In '98, Terrell Davis had become only the 4th RB in the history of the league to rush for over 2,000 yards. Dude averaged 125 yds/game

and of course was the centerpiece of the 2-time champs.
Cunningham had a great year, but his stats weren't anything special - 5th in yards, 6th in completion %, second in TDs. Those aren't MVP numbers let alone good enough to eclipse what Davis did that year.
Davis deserved the MVP for breaking 2,000 just like Barry, dikkerson and OJ before him. The only reason dikkerson didn't win was because of Marino's season - still the most prolific passing season to this day.
In 2000, Marshall Faulk had once again gained over 2,000 yards from scrimmage and had
26 TDs, the most ever by a non-QB at that point.
Culpepper finished 4th in yards, 4th in completion % and 4th in QBR. He led in only one category (TDs) and threw 16 INTs.
How were these snubs?