[rcxdude]

From experience linear controls is probably more popular because there's a larger set of tools in the toolbox, so to speak, and the nature of the systems is that this is (usually) close enough for modelling nonlinear systems which are close enough to linear (in fact you may introduce nonlinear terms in your controller to try to remove nonlinearities in your system). It's not just having a 'proof' of stability, it's having methods to analyse the system and optimise the controller, which are much more difficult to apply in general to non-linear systems. It's much like how engineers will generally just stick to classical models of physics despite them being 'wrong', because in practice they are close enough and also much easier to deal with.