Using the English alphabet, they were going to run out of letters. There’s always been a numeric version to every Android release, but maybe they wanted to reserve the possibility of skipping a number, as Windows did going from Windows 8 to Windows 10 (what the heck is wrong with the number 9? Apple skipped it for iPhone as well..). Surely it is not the Curse of the Ninth?:
https://en.wikipedia.org/wiki/Curse_of_the_ninth
If so Google has already been cursed.
I hate Roman numerals. I bet even the Romans hated them. They should never ever ever be used again. It might have been an honest mistake, but it is one of the first things I would want to correct when time travel becomes broadly possible.
In keeping with the times, perhaps there should be an emoji for every release.