Indeed, Berzerk with more enemies and dual sticks would be like Robotron with walls. Maybe one could see Robotron as a sequel in spirit.
About the 2600: I am pretty sure the sprites are the reason. Existing 2600 games have flicker problems already, but Robotron quickly has 50+ moving objects. The same problem goes for NES and SMS to a lesser degree. Those work very different from the 7800 for example, and while they usually have the upper hand when it comes to level of detail, they are not good at handling lots of sprites.
IIRC there was a Robotron 2600 experiment on AA, some homebrewer tried to get so many sprites working. There's a thread somewhere...
And Robotron C64 is indeed disappointing. Much too slow. Seeing how less powerful systems hhad better versions is a shame.
Robotron X/64 looks so much like I would imagine a Jaguar Robotron 2000 by Jeff Minter, at least in style. Istill hope Reboot will someday do a Rebootron.