1. Because Public Enemy has a much more immediate rock influence on their sound than most Hip-Hop
2. Because you've been looking for other good Hip-Hop on the radio, MTV, BET, and other mainstream outlets, and 90% of it outright sucks. Most of the good music is independent (and I don't think this is just true of Hip-Hop)
In my opinion, there is Hip-Hop being made that is as good as ever, but it's not what is popular. Maybe check out these albums from recent years:
El-P - I'll Sleep When You're Dead
Blu & Exile - Below the Heavens
Cunninlynguists - A Piece of Strange
The Roots - Rising Down
Aesop Rock - None Shall Pass
Lupe Fiasco - The Cool
Cage - Hell's Winter
K-Os - Joyful Rebellion
Zion I - True & Livin
K'naan - The Dusty Foot Philosopher
The Mighty Underdogs - Droppin' Science Fiction
Nas - Untitled
Q-Tip - The Renaissance
Black Milk - Tronic
Cee-Lo - The Soul Machine
Brother Ali - Shadows On the Sun
Panacea - A Mind on a Ship Through Time
Pharoahe Monch - Desire
Madvillain - Madvillainy
Atmosphere - When Life Gives You Lemons, Paint That crap Gold
El-P and the Roots are probably the first places to go from Public Enemy.