Well they did find Shmi, but by then she'd been freed and married to Cleig Lars. She did free a shitload of other slaves from Tatooine, but at that point there's no reason to steal Shmi away from a reasonable life.
She wasn't a queen in the traditional sense, it's an elected position on Naboo. Hell, even on Earth it's not necessarily an omnipotent position. Just because they hold a high position doesn't mean they can wield that power for whatever whims they want. They have a whole planet to take care of, first.
Revolutions tend to overthrow monarchies that ignore the needs of their populace.
I mean... C'mon though dude, we're literally talking about ONE slave here. If you don't have the political capital and/or diplomatic savvy to secure the freedom of a single, solitary piece of human chattel from an old, racist stereotype alien then are you even really a politician at all?
Besides, if memory serves (which, granted, it's been awhile), wasn't Watto pretty taken with the sweet, chromed out hot rod they waltz in on to begin with? Cuz I'm no businessman or anything, but I'm reasonably sure there's some sort of deal to be made there...
Provided he would even be willing to trade that. His entire business seemed to depend entirely on Anakin (who was taken from him), so holding on to the one slave he had left, even if just out of spite, was probably more important to him than a little bit of bling that would be fleeting.
He was doing business ling before Anakin was ever born. He was paid well for Anakin. His business didn't depend on Anakin tinkering with broken robots.
She didn't need to conquer an entire planet. Qui-gon bought a slave with the earnings from one pod race. She could have pawned one of her dresses for more money than Watto had ever seen. He was literally living in a dump.