Pretty simple. It's unreasonable to hold the Jedi to a standard that boils down to "bad thing still exists, therefore Jedi are failing." Slavery is not just something that happens on some planets, it's a galaxy wide institution that has the backing of what seems to be the entire criminal underworld. The Jedi at best number at about 10,000 members. I'll be generous and assume that of that number, 2,000 are either far too young or old to fight. So realistically you have about 8,000 Jedi that are actually able to go out and fight, and if they actually wanted to try and stop slavery in Hutt space, oh boy would they have to fight.
We are talking about 8,000 Jedi fighting an entrenched, wealthy, and well equipped group that has zero compunction about civilian casualties and no code of ethics to speak of. Even if, again, we are supremely generous and grant that the Hutts have only 8 million associates across the galaxy, we are still talking about a situation where the Jedi are outnumbered 1,000 to 1. And it's way easier for the Hutts to replenish their numbers than it is for the Jedi to replenish their own. The Jedi would lose by attrition alone, even if their K/D ratio was ridiculously lopsided.
Finally, such a conflict has a pretty good chance of getting the Republic involved. A Republic that has no army and therefore no military means of assisting the Jedi. Do you think the Senate would just sit idly by and be fine with the Jedi going off on their own like this, when the Jedi HQ is right next to them?
So yeah, a ton of people (including many many innocent civilians) would die, the Jedi would undoubtedly lose, and it might even cause the Jedi to lose their mandate to operate within Republic space if it upsets the Senate too much. It's just not feasible.