It's do to the content. They don't want the gritty netflix heroes crossing over with their movie heroes who are marketed towards children.

It's probably business related but I'm not sure a crossover would even be a good idea for artistic reasons. The Avengers are super-people above common trivialities, who spend their time resolving global crisis after global crisis. Daredevil and Jessica Jones are average, working-class folk dealing with the fallout of the Avengers' heroics. I don't wonder why Iron Man hasn't shown up in Daredevil because it would be downright bizarre if he did.

Maybe they are too noir or dark to be shown on the MCU. Daredevil and Iron Fist were bad calls to make into a netflix series.

Political backbiting between the studios responsible.

Daredevil's comic counterpart has never been sunshine and rainbows, silly name and costume notwithstanding.