The short is answer is that EA Sports paid $300 million for the exclusive NFL license in December 2004.
The common belief is that a combination of the quality of ESPN NFL 2K5 and its low price tag ($20 instead of the standard $50) put a big dent in the sales of Madden NFL 2005, and EA wanted to shut 2K out of the NFL gaming market.
When the news broke, NFL’s stance was that they wanted to partner with a single publisher to better represent the NFL brand. That’s nonsense. It was the $300 million that swayed them. But, for what it’s worth, the deal led to NFL having more input in Madden games. Their influence got EA to tone down extra violence, sexy cheerleaders, mentions of concussions, or anything else that might paint the NFL in a bad light.
2K wasn’t making college football games or any other football games at the time, so they just stopped altogether for three years, then released All-Pro Football 2K8 using made-up teams and retired players, and that was the last 2K football game to date.
News broke in March 2020 that the NFL is opening the license back up to other publishers and that 2K is planning a new NFL game. However, the new 2K game is not currently planned to be a simulation-style football game.