I think it's because the source-material company (Milestone) would have an issue with DC/CartoonNetwork/etc making money from their work.
You're right: getting the air-time does increase the knowledge-of and popularity of their stuff. After all, Static and such are not as well known as other characters. Outside of the die-hard comic fans and experts, the only people that know about them a little bit are those that watched the Static Shock cartoon some years back.
That's one of the consequences of Trademarks... they keep people from profiting off your property without your permission (and a payment). Also so you keep control of your IP and ensure it's not misrepresented or depicted in a way that might hurt your bottom line.
For example, without Trademarks and Copyright... some random company could make a rated-R "Superman" cartoon where he's a rapist. Or a rated-R Batman series where Bruce Wayne is actually a full-blown pedophile and is having an illegal relationship with Robin.
Which are things DC would prefer NOT to air, since young kids are a core audience and I imagine the above might hurt their sales... just slightly.
Edited on 12/10/2012 6:34am
Edited 3 total times.