I think Magnus just changed focus to being the first 2900. I bet the way the last match went was a bit of a let down for him as well.
By resources i mean things like going into a position that gives your opponent an incredibly small advantage but is hard for them to play. It's one of the best ways to win games. it's hard work to find those lines, and he's been doing it for so long at WCC level it's not surprising that he's tapping out against someone he's already beaten. if you use a line in the WCC, it becomes very risky to use it again - because if your opponent studied it and plays it correctly then all you've done is make it even harder for you to win the game.
to reach 2900 he has to win games, not tie
That doesn't exist anymore, any game Carlsen plays is studied in depth by the world immediately, and especially by his opponents. Same for other top grandmasters.
Many observers overplay the importance of one match loss. More than one chess champion title was taken by a challenger who had lost in the previous attempt [1] - and considering there was just a handful of champions, it is a lot.
[1]: Spassky and Kasparov have lost in their first attempts against Petrosyan and Karpov.
Magnus specifically mentioned Firouzja as an opponent that he would consider playing the WCC against[1]. So while not the most important factor in his decision, the fact that Nepo won the candidates again and was the last challenger before Magnus made the Firouzja statement seems relevant. Also relevant is that the candidates tournament concluded on July 5th and Magnus released his withdrawal now in the same month.
It seems to me like the challenger being Nepo was the last significant piece of information towards his decision. How specific it is to Nepo or a more general feeling of Nepo being one of many people he doesnt feel any personal benefit from playing against, who knows.
[1] https://chess24.com/en/read/news/magnus-carlsen-unlikely-i-w...
That is not how it works, and not how it is supposed to work. This is hubris and disrespect to all chess players in the world. For more than 100 years champions changed each other in title matches (well, before Kasparov's limitless ego created a decade of turmoil in Chess), with just one exception by Fischer, and now Carlsen 'doesnt feel any personal benefit'. :ThumbDown:
So it isnt the most important reason, but nepo being the challenger didnt offer anything to change his mind like he was hoping the result of the candidates would.