I imagine some of this was because a lot of Dems voted for Nilkki Haley to tweak Trump. Indy is open primary. But that can’t explain all of it.
Yes there was crossover sure but even if you add ALL of Nikki’s votes to Biden’s (which isn’t going to be reality but even if you did) you still get the same result. Biden just underperformed every Democrat primary winner in Indiana in more than 30 years