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 an absolute dumpster fire but in no way compares to the awfulness of ME.
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.