I think you need to understand that most people maybe had 16~32MB of system RAM on average circa 1999, while Windows 2000 ideally wanted at least a staggering 128MB as just a sysreq recommendation. Windows 98 was fine with 16MB minimum or even 8MB with some wizardry.
How much did 128MB of RAM cost in 1999? According to a Reddit thread that Copilot found, about $370 in 2020 dollars[1]. Copilot also dug up another thread[2] where people reminisced about RAM capacity and pricing in the 90s.
So in very basic terms, 128MB of RAM (let alone more!) back then would be like buying 128GB of RAM today. Most people simply aren't going to buy ~$400 of RAM just like that, especially when upgrade cycles were also much shorter than today.
There was a very practical reason Windows 9x existed alongside Windows NT until XP merged the lines in 2001 when 256~512MB of RAM and more became much more affordable.
[1]: https://www.reddit.com/r/mac/comments/finsjm/how_much_did_12...
[2]: https://www.vogons.org/viewtopic.php?t=62239