What is the technical reason why Interstate75 can only drive up to 256x64?

I am trying to get a lot of Hub75 panels driven. I have therefore purchased several Interstate75W Picos.
I wonder why I can’t connect more than four 64x64 panels to one Pico. Is it a timing issue, a memory issue, or something else about how the panels work?

Well - it turns out that it can drive more panels, at least with the C code. I connected 6 in row and created a Hub75 instance with 384 by 64 px, and it just worked - I can access every pixel in it. The only problem is that I’m slowly running out of RAM with the other things I want to do on the Pico …