The reasons Farsight broke away in the first place are pretty straightforward. During an expedition outside of the Tau Empire, he encountered a previously unknown species (Chaos daemons) and his Ethereal handlers were killed. Free from their direct influence and aware that they were manipulating him and the rest of the Tau, he was understandably not inclined to go back.
As for why the Ethereals haven't gone after him, it's probably because in the grand scheme of things, he's not that big of a threat. Farsight has no designs on conquering the Tau Empire itself, and his forces have fought alongside the regular Tau against foes like Hive Fleet Kraken and the Imperium (during the Second Agrellan Campaign). Since he's willing to fight their enemies but not the Tau, Farsight makes for an ideal buffer state to help shore up the Tau against any threats. Plus, any innovations he makes to weapons, tactics, or technology might be of value to them in the future.
The other consideration is that Farsight isn't likely to come back without a fight. Whatever mind control the Ethereals may or may not use on other Tau, it certainly won't work on a foe who knows how to avoid it (mainly by avoiding contact with the Ethereals themselves). They would need to invade the Enclaves which, although they would probably win, would take valuable resources away from the empire's other border conflicts. As long as Farsight isn't causing too much trouble in the empire proper, there's no reason to go to such lengths.
