There are very few areas in the developed world that are not covered, and few competitors left to buy out (in the UK, we're down to one major cable provider), so the easy gains where they'd roll into a new area and find a substantial proportion of residents waiting eagerly to be able to get cable service, are gone. Now they have to compete for customers that already have some other service they have actively chosen despite the availability of cable, and similarly face losing customers to those same services, both of which makes continued growth much more difficult.