Because in his mind the advancement of the Democrat party agenda is the most important thing in the world, and any cause/organisation must be sacrificed for that goal.
It is a mirror image of the NRA expressing an opinion on ObamaCare (which they resisted doing. I don't remember if they ended up caving to the pressure from Republicans).
The NRA is actually a perfect example. Whether or not they issued an official opinion on Obamacare, their TV station and spokesmen constantly speak about right-wing social issues that have nothing to do with gun rights. Additionally, they were very conspicuously silent in defending legal gun owners in certain cases, like Philando Castile*, which didn't fit the perfect right-wing narrative.
*They eventually did say something, more than a month later after tons of backlash. And even then, not even close to how they spoke about other cases.