Because Friends ran its course.
The story was completed: Ross and Rachel ended up together, Monica and Chandler have children, Phoebe and her boyfriend were planning to follow up and Joey was moving to Hollywood. There was nothing more to say, the group of young, carefree friends was no more - they were now in their 30s with own families.
The out of universe reason is... that the actors and producers had enough. Even the final, 10th season almost didn't happen -Jennifer Aniston was already threatening to leave and the producers David Crane and Marta Kauffman were seriously wondering "what else there is to tell":
Kauffman and Crane said they wouldn't have gone another year even if all the cast members had wanted to continue. "It took us a while to get onboard with the idea of season 10," Crane said. "We had to really sort of examine what stories we have left to tell that would justify coming back. I'm glad we did, because I really liked this season a lot. But you don't want to overstay your welcome."Source
There are more rumoured reasons revolving around the costs of production (towards the end, the main cast was earning over 1 million per episode, in compare to 22k in season 1) and the general change of the trends in TV: in 2004 sitcoms were becoming less popular while dramas like House or CSI started taking over.
Overall, it was best for everyone to just finish the production and keep earning money from never-ending re-runs.