Several probable reasons:
1) You can't sing
2) You (or any of your friends) can't compose good music
3) You can't dance
4) You don't have style
5) You aren't original/creative
6) You can't rhyme
...

Several probable reasons:
1) You can't sing
2) You (or any of your friends) can't compose good music
3) You can't dance
4) You don't have style
5) You aren't original/creative
6) You can't rhyme
7) You don't have access to a recording studio
8) People don't like you
9) You don't know enough people with connections
10) You don't have enough money to fix any of this

You dont have lot many qualities which a successful rappper must have. Go and learn about it from one of them.

Good Luck.

the first thing I can think of is uh....you suck...