He doesnt throw leg kicks because they are not that good. He had some success in their second fight because Nate has not learned how to properly defend them despite a decade long career. Tony has fought RDA and Barboza, both are very hard kickers and they didnt have success against Tony. Not only that but low kicks would leave Conor vulnerable to takedowns, in fact Nate took him down after catching a kick.
Conor's strategy will be to stay at range and aggressively counter Tony, looking for an early (T)KO, which is the same strategy he's used against everybody