I note that you are now changing the discussion from the number of double-digit losses to the exact number of the double-digit. Izzo lost to a 15, Knight to a 14. To be honest, I really don't see that much of a difference. It's a big upset. I realize that you want to make a big deal of it but I honestly do not see significant difference between a 14, a 15 and a 16 seeded team. It is a large loss and the distinction is rather small in my opinion.
As for FDU, I recall that the team that they were playing in the one-bid league that they were in at the time was prohibited from advancing to the NCAA because they were new to the league. To me, that was likely a factor in FDU even getting to the tournament as I strongly suspect that the team that they were playing in the conference final was likely affected by knowing that even if they won that they would not advance.
Painter being unable to advance to the FF prior to last year is a combination of a number of factors. Talent had spread out prior to NIL and that made the tournament more wide-open. More lower-seeded team advanced, George Mason anyone? On top of that, Painter had some bad luck with injuries to key players: Hummel, Haas. I don't know how he could have been blamed for those. Added to that was internal issues at Purdue with economic constraints placed on his program that nearly caused him to bolt elsewhere. The combination worked against him.
You use RMK and Izzo as examples and I think that is not a valid . RMK had the university behind him to an extent that Painter did not. Historically, IU placed an emphasis on basketball and not football. It was one of the rare cases where the BB coach was paid more than the FB coach. I understand that is not true now, but we are referring to the past. That was not, and is not, true at Purdue. At MSU, while favoring FB, did not have the gap in program assets that Purdue had. Assets and success are positively correlated.
Remember, they teach you in Math class that you cannot compare apples and oranges. In comparing programs at the time, they were different kinds of fruit for the reasons mentioned.