'Cause they have an army. Look what happened when the Sith had a bigger one. Sidious took over the galaxy. Revan would have if Malak wasn't a complete and total idiot.
See above.
The nature of the dark side is destruction. The strong survive (Revan, Bane), the weak die (Malak, Retard).
The Jedi are stubborn stuck-ups that inherently believe they're stronger than everyone else.
The Republic is perfectly happy to have them pull the strings in the background.
"...Valorum [was] appointed the first non-Jedi Chanellor in over four centuries." (Bane: RoT (95)) Jedi ruled the Senate for 400 years.
They didn't allow the Rakata to leave Lehon, thus aecuring the species' extention.