For TV networks in the USA since most Americans have short attention spans, no wonder they watch that crap called American Football where play is stopped every 10 seconds. I never had any coverage problems when AMS events had best of 5 finals.
Play frequently stops in less than 10 seconds in a tennis match. Do you have a short attention span too?
It's about scheduling, and this problem exists for all TV networks, including those outside the US. If a long tennis match (or any program) ends up extending into another program, then the TV network doesn't make as much money. Therefore, TV networks aren't as willing to shell out as much money to broadcast events with indefinite lengths. It'll still get shown, but the tournament/ATP won't make as much money. It's actually pretty simple.