Let's be honest: Supergirl (Kara Zor-el, at least) is used more often than not as rape-bait (Libra to Lex Luthor in Final Crisis: "Help me and you will be the first in line with Supergirl"; Metallo when depowering and killing kryptonians, speaking about Supergirl: "That's an S I really want to see without powers", and so on; although that last one was written by Scott Lobdell, who's famous for this).
People often have a hard time writting stories about a male invulnerable almost god-like being, and that's with Superman having a very iconic support cast.
Get a bunch of single, straight white men to writte stories about a female god-like being with no previous support cast and, well... Has there ever been any really great Supergirl story arc, other than the ones with the Matrix Supergirl?
I rememeber the beginning of the New 52 Supergirl, with such pearls of good writting as "meet Kara's new major villain! Do you like his chest window?". Then they made her into a single minded neally berzerker character, and when that failed they made her a Red Lantern. Really, it's not going well.
Green Arrow has an interesting supporting cast. Flash has nice-looking powers and maybe the third best gallery of villains in comics. SHIELD is linked to nearly everything in the Marvel Universe.
Supergirl has... What? The TV show's supporting cast is either stolen from Superman (Olsen) or a stereotype (the "friend who's in love but the main character is the only person who doesn't realize it" thing). But honestly, could they do any better? Is Supergirl (that one, at least) a strong enough characters to hold a TV show?
Because her comics don't look like so.