Depending on where you draw the line for millennials the school computers could have been running anything from Windows 95 to Windows XP. BASIC wouldn't have been included in XP or 2000, and depending on what exactly they were using, it may have been easier to use something with more features. QBasic for example wouldn't have syntax highlighting. Simple programs in languages like python or perl can be used to teach the same concepts, with fairly similar syntax, and would have been more applicable to software people were actually writing at the time.

If you define a millennial as someone born from 1982-1996, the majority of those kids by the time the had a computer lab (say by the time they were in middle school) would have been too old for BASIC. You are talking 92-06, Windows 95 took over the majority market share by the end of '96, and you are forgetting a whole lot of schools had Apples being pumped in, so a lot less access to BASIC. Unless you were in a programming adjacent class, the majority of millennials weren't learning basic. Ask your non tech millenials how much BASIC they learned and they would look at you like you grew an extra arm.