You get barely a few hundred watching the streams and you would get a few thousands watching it on TV, which is not an audience any sponsor would pay for.
Challengers happen in smaller halls and clubs, it's not like watching Chatrier. Anyone who has watched a challenger knows that only a few are worth to be seen, also because of the bad tennis level there.
When you're used to watch the big guys you could suicide from boredom watching the actual mugs of the game.
We get bored watching Tipsarevic, Ferrer, etc. and we are hardcore fans, what to say about their lesser versions...
Not even the betting sites are showing this tennis and they have both the possibility and interest, but obviously not enough interest compared to even Philipino Basketbal..
Betting sites stream Challenger tennis pretty much every day of the year.