There are many fewer DINOs than RINOs for two reasons:

One, because the Democrat Party is inherently collectivist. You don't usually get far enough in it to see public office if you aren't already part of the groupthink.

Two, the political environment in Washington has become leftist and collectivist. If you want to do anything outside their worldview without being confronted at every turn, you have to "go along to get along" to some degree. Even principled centrists and right wingers can get beaten down, facing this constant opposition, so very few are able to succeed without some compromising of their values, and those who are just there to gain power simply give in every time while decrying the corruption to their constituents back home when they need to be re-elected.