Personally, I don't think solresol has failed because it's perceived as 'gimmicky.' I think in the right environment any musical conlang could succeed. That's the main thing that I think is missing; an environment that properly endorses the development of the speakers. Personally I think Moss is much more sound in its phonology, so I'm more likely to endorse a Moss development community than a Solresol community, but I take issues with every musical conlang I've seen and that's why I'm making my own. This is neither here nor there. I think what's more relevant to your point is 'the reason they haven't caught on.' Yet I would argue the same thing about Esperanto; the only people I know who speak Esperanto are folks somehow trained in linguistics. I struggle to believe you will ever get average folk to learn an entirely new language unless they are forced to by unpleasant forces...or something irresistably attractive. Linguists are attracted to learning, as are musicians. If I were to make a case for a musical conlang, that would be it. There has to be something irresistable attractive about it, and it has to be in the community more than anything. There has to be something that draw people into the learning.