They were developed by completely different teams.
As an aside - as someone who used ME back in the day, I feel like I honestly had more problems with Vista. ME was a downgrade from 98SE for sure, but I don't remember it being the same level of performance and reliability degradation that I saw going from XP to Vista pre-SP2.
Vista was fine from the get-go if you had enough (>=4GB) RAM, which OEMs mostly didn't bother shipping.
My ME machine would reliably BSOD when I opened / closed the CD tray.
My first (and only) experience with Vista was with a stripped-back Toshiba Satellite A135 my mom bought my brother and I during some Black Friday sale. It had one single 512MB RAM stick. I still have a screenshot kicking around somewhere of the "Windows Experience Index" of 1.0 or 1.5 or something (1.0 was the lowest) that also shows the RAM amount. We made it work, though. Many good memories of recoding Xbox360 footage using some Lexar capture box that only accepted analog RCA in as the laptop cooked itself alive sitting on the carpet in front of the TV.
I bought a machine with similar capability for my wife that shipped with Vista and it was literally unusable. I think I ended up "upgrading" it to XP.
I had a big problem with BSODs caused by Nvidia drivers. Of course, you could argue that this was Nvidia's fault, not Vista's, but this was somewhat academic. I moved back to XP (and also started using Linux) and all these problems went away, and I got a lot more out of my RAM to boot.
Vista was an absolute dumpster fire but in no way compares to the awfulness of ME.
Vista wasn't remotely a dumpster fire. I used it for almost its entire lifetime, it was totally fine.
People did all the XP customization with compositor 3d effects and matrix screensavers. Then they hyped up Longhorn with all the magazines and delivered the Vista dumpster fire.
IIRC ME would just randomly crash explorer.exe with task bar disappearing but you could recover some of it. On Vista it was just overall sluggish and laggy visual experience.