Michael Piller wanted to focus and deelop a few signature races but his attention was divided between Voyager and his other series-Legend.
So he wasn't there to push the issue. When Jeri Taylor got more control she didn't want to do that because she felt they were moving through space to get home and wouldn't be spending that much time in any given territory or region.
Another consideration she had was the cost of designing and developing the race's makeup costumes etc. Brannon Braga shared Jeri's opinion of more one-off species than spending too long on any one race.