Why can't knuckles tails and shadow be playable like sonic in brawl I mean there are 6 Mario characters playable in brawl and there is only 1 playable sonic character so why can't they be playable?

The developers of the game do not own those Intellectual Properties, so they may not have been allowed to use those characters. Also, the developers are limited by time and budget, so they may not have been included due to lack of funds and time.