Long story short, because of money. According to this article Universal TV wanted to produce a new season but Fox network was spending $5 million per episode and wanted a cut in the show's license before renewing. They couldn't agree on an amount. "Universal came off as aggressive," one exec says.

You can see this in the ending montage. It was used to prop up a table in Foreman's office which he was having a problem with earlier in the finale. House is the only person that knew Foreman was having this problem so Foreman immediately smirked and realized that House was still alive.

He then returns and cures the daughter/wife/some other relative. The rich guy then decides to start a private medical consultancy employing House, a place for other doctors from all over the world with the most Bizarre cases ever to come and seek House's help. He finally ends with Dominika.

Cuddy left the show following the events of House MD season 7's finale, which involved a brutal breakup with House. ... Most of the actors' contracts ended with House MD season 7, and the network wanted to cut down on costs by bringing them back with pay cuts. Lisa Edelstein refused and decided to leave the show instead.