Because they haven't released a game in 5 years and Nintendo didn't seem to blink to give them Prime 4.
I made some research on this topic this morning, little recap for you (i won't post links again but you can find everything on the internet):
- there were concrete rumors in 2013 that several employees, most of the veterans would have left the company after Tropical Freeze, and Nintendo had to essentially rebuild the studio.
- we know for sure they started working on something in 2014 (they admitted it and Tanabe was involved), but the way Tanabe talked about Retro the year after, he made it look like that project was already gone (or maybe not reading Eric Kozlowsky's tweet, who btw left the studio in 2015).
- Since then we haven't heard any news until they started hiring again by the end of 2016 / 2017.
If you listen the interview Game Informer did with Mark Pacini a couple of years ago he said before Prime 1, there were several iterations of teams at Retro Studios that came and went, so at some point they had maybe the best people from all these previous teams.
Also i remember previous interviews where Retro's CEO said it takes time to teach western developers to think like Nintendo to develop the kind of games they want to see, which makes me believe in these 5 years Retro faced a transitional period, training these new employees, we don't know whether from this period they still managed to make a game or not but i wouldn't take that for granted.
At least now we know they wil work on Metroid Prime 4, that's already better than those previous years of dead silence.
