Because he is boring -_- his color is bland, his design other than his third form is very bleh, and he just sits around in his throne-room all the time until all of his minions have been beaten. If it were King Piccolo he'd be going out and getting crap done. He wouldn't be sitting around just watching things and then taking forever to fight.
Frieza's fight alone was bad enough with how ****ing long it was, but add to it the long ass time it took to get to his fight with how boring Namek was and that whole Saga's practically unwatchable to me.
Frieza is a galactic overlord who controls his pawns from the throne and plans the next move after another. Thats what evil leaders do, ALOT of anime villians do so. Plus the fight with frieza was originally supposed to be the last fight of the series, but fans kept bugging toriyama to continue the story. Not to mention toriyama likes to make the most powerful characters look simple. Frieza's final form, Super Sayain God, Kid Buu?!?! All of their designs are simple.
King Piccolo didn't. He got in there and took care of things himself once his minions started falling. With Frieza he just lounges around while EVERY SINGLE MINION gets killed off.
As for the end of the series, Toriyama wanted to end it at basically EVERY part of the series. First Pilaf, then Piccolo (which is where I would've wanted it to end), then Frieza, then Cell, the finally Buu (and personally...it's rediculous that anything can even go beyond Buu).
But, years later, Toriyama made Beerus, who's even more overpowered than Buu was. And two years after that, he decided that Freeza should get an extremely ridiculous power boost to match up with SSJ God Goku. If this continues, we might see an overpowered Cell return.