The phenomenon is that consumer prices for broadband go up, even as the wholesale cost of bandwidth (what Comcast is reselling to you) goes down. The cause of this is a mix of two things:

1. Cable companies have regional monopolies. Without a choice, they can raise prices on consumers without losing customers

2. Cable companies face ever increasing 'infrastructural' costs, such as environmental review, trenching, construction, permitting, easements, etc... These costs are well established as going up in excess of inflation (part of why we can't build new infrastructure anymore) and therefore even while the product itself (internet access) gets cheaper, the 'last mile' of distribution keeps getting more expensive.