When you consider the liqueurs, fruit purees, fruit curds, and the chocolates, as well as combos like chocolate-grand marnier, chocolate-frangelico, white chocolate-chambord, there are actually quite a lot of options for mousseline! But I think the real answer to your question is that some flavorings taste better with certain buttercream bases than others.
Take the lemon, for example. Lemon juice and zest are wonderful with the richer base of the classic/neoclassic, and lemon curd is wonderful with the mousseline base. But once, I added lemon juice and zest to the mousseline, and although it worked just fine, it wasn’t nearly as good as the other two options.