The reason they've lost so many games?
15 total losses.
You can subtract any games Martinez didn't play or left with the team up and any two-score game because that's a team loss. I understand a great QB can overcome multiple scores, but that's on the team, both falling behind by multiple scores and overcoming it.
6 losses remain.
Iowa '19 - Iowa gave up an average of 14 PPG in 2019. Nebraska lost 27-24, but had a non-offensive score. Long scores due to failed containment, poor special teams, a killer Mike Williams "penalty", 2AM failing to stay in bounds, long play given up with seconds remaining, a dagger kick from an All-American kicker.
Martinez played poorly. This loss could be pinned on him, but there are other obvious factors.
Iowa '18 - Iowa gave up an average of 18.1 PPG in 2018. Nebraska lost 31-28. Large deficit most of the game, big brass balls call to go for it from inside NU's own 10 after stopping Iowa on 4th down and goal changed everything, and a 4th down conversion for Iowa preceding a walk-off FG.
Martinez had a passer rating of 138, a 68.5% completion percentage, and a 2:1 TD:INT ratio while accounting for 336 yards. This one's not on 2AM.
Purdue '19 - Missed opportunities in the red zone, a late TD drive by Purdue's 3rd string QB was key.
Martinez accounts for over 300 yards, but plays poorly. I would put this on the defense failing to get the stop, but I can see this one being put on Martinez due to the failure to score in the red zone and a terrible INT after a blocked punt.
Colorado '19 - Backup punter misses potential game-tying FG, CTB bites on a flea-flicker, team played flat after the half.
Martinez accounts for over 350 yards, 4 TDs, and a passer rating of 172.9. This one's also not on Martinez.
Ohio State '18 - Nebraska leads at the half, but gets bullied in the second half. 2AM has a bad lateral that likely cost the team at least 3 points in a 5 point loss. JD Spielman drops a crucial walk-in TD pass.
Martinez accounts for over 330 yards and 3 total TDs with 0 INTs in one of the toughest places to play in the country. If you're hanging the kid for not playing perfect, go for it, but he played well here.
Northwestern '18 - Wildcats come back from down 10 with about 3 minutes left in the game and convert multiple 4th downs to do so. Martinez throws a pick from the 40-yard line in OT on 4th down while running from pressure.
Martinez only accounts for 288 yards and this game is one of two in which he throws multiple INTs in a loss (2019's 48-7 loss vs. OSU is the other). If you watched this game you know 2AM wasn't the reason for the loss.
So, there's debatably two or three losses that you can hang on Martinez if you're so inclined to ignore the fact that football is a team game. You point to a 9-15 record, which nobody is happy about, while claiming that a kid who accounted for 10 points in 10 quarters is better than a kid who's statistically been better at his position than anyone ever to attend the university through two years and was voted a team captain as a true sophomore. Your stance is simply indefensible...but don't let me or anyone stop you from trying.