Acquisition costs for used MD90s are CHEAP.

The MD90/737NG/A320 are all from the same era. Operating costs of an IAE V2500-powered MD90 are essentially the same as a V2500-powered A320. All MD90s are V2500-powered.

Just because it was built by McDonnell Douglas doesn't mean it's some antiquated, fuel-guzzling, flying contraption. MD90s and MD11s were being built into the 2000s.

The only reason Boeing ended MD90 production was because it directly conflicted with the 737-800. The same reason holds for the MD11, which was put to pasture by Boeing so as to not compete with the 777.