Oh, let me count the ways:
1) Because TV series isn't for B-string characters.
Gregory House. Rick Grimes. Walter White. Tony Soprano. Daenerys Targaryen. M'fing Jack Bauer. I could go on, but I think you'll find that some of the best fictional characters ever make their home on TV.
2) TV is better for comic book storytelling
While there's a lot to be said for movie spectacle, a typical comic book story arc can't be told in 2 and a half hours, and over the course of a trilogy, you only get to tell three stories, only the character's most classic beloved ones. But what if a character has 10 reat stories, like Daredevil does? Oh well, tough cookies, these are the movies. A TV show can tell all the Daredevil stories. You will get at least 13 hours of Daredevil in one year and not, y'know 6 at best over 20 years.
3) Daredevil doesn't need a movie budget
Matt Murdock doesn't have a big suit of armor with missiles. He doesn't fly. He doesn't summon lightning and travel through dimensions. He doesn't fight super Nazis, he doesn't turn into a huge green monster, or travel through space with an alien raccoon. He fights criminals. With guns. And sometimes with low-level powers light super strength. That simply doesn't cost 150M dollars to film, as every action TV show ever will attest to.
4-6) Jessica Jones, Luke Cage and Iron Fist are no different.
They are also someone's favorite character, and Jessica Jones is just as award winning, iirc. They also can be done awesomely on a TV budget.
7) Daredevil isn't actually that special.
Daredevil is one of about 20 incredible pivotal characters that Marvel has. If they try to do all of them as movies, then we might get a Daredevil movie in 2022. Maybe. And we may never get some of Marvel's other awesome characters who can never be done justice on TV.
This is the right movie, and DD will be better for it.