Giving so many customers 300Mb upload means it takes only a few customers uploading large files on a node to congest it, including business customers.
Or all the people torrenting, running Plex servers, and who knows what else.
While most customers don’t do heavy uploading, all it takes is a few to cause congestion at those speeds.
I have no idea if that’s the reason, but that’s the only logical one I can think of.
Other cable companies are more conservative with their mid-split upload speeds.