It's mostly technical as WiderMouthOpen mentioned. The cable network was designed around the concept of prioritizing download speeds. In fact the very first cable modems didn't even have upload capabilities at all. Upload was done via dial-up modem and download was done by the cable modem!
But it's also a marketing reason in that cable internet was originally designed to be an entertainment consumption service, which requires a lot of download speed. Upload was mainly used to request the next web page or the next video. Nowadays people use their internet differently but a lot of cable operators still see themselves as providing an entertainment service that has no real use for upload speeds. Not to mention that there is a stigma attached to high upload speeds because that can be used to distribute pirated media.
