Detroit seems to have a lot of elements in common with the other early hotspots of hip hop, and even had a strong musical background with Motown. But Detroit wasn't really a big part of the scene until Eminem came along in the early 2000s, and even he had to go out to LA before he could do much with music.
So why did Detroit not contribute as much to early hip hop as NYC and LA, despite having all the right ingredients?
It's not really true. Detroit was the og home of techno/house and breaking music before hip hop, as we'd define it, existed. In the 80s, detroit was the underground electonic music capital of America. The transition to hip hop happened with the rest of the nation. Obviously it was a big part of motown and r&b, which also influenced the musical culture.
There is still a lot of crossover and production influence - look at j dilla for example. But for the most part, Detroit hasn't been a dominant metropolitan area for a long time, so it is a cultural follower rather than a cultural leader - obviously not to say that the city's culture isn't a unique blend.
In the end hough, the city is known for afro-house/underground techo in the 8p 0s/90s - that's the shortest answer as to why there wasn't a leading rap community. Obviously rap was popular and common - but it was part of a national movement, whereas Detroit techno was always homegrown.