Linguist here. It really isn't. There are plenty of competing and very compelling theories in e.g. big parts of Europe. Especially Functional Grammar and its different branches. The only relation they really have to UG is that they're not it.

At my university no one in the linguistics department actually subscribed to UG or any derivatives thereof but in the English department they did. I've observed sort of the same divide in countries. In English speaking counties UG seems to be the prevailing theory while it's much less so in non-English speaking countries.

Unfortunately people are extremely religious about their preferred theory of grammar, and it's hard to have a useful discussion about it when people are unable to remove themselves from the subject and discuss it objectively.

I studied theoretical linguistics through the graduate level and it wasn't until rather late in my schooling that I even began to hear of competing frameworks such as HPSG or LFG, and much of it in passing. There was no mention of them in the coursework, and the tendency of faculty to sell modern linguistics as a hard science gave the impression that we were learning a standard approach that had a provable empirical edge and broad scientific consensus. Otherwise, why would it be called "Extended Standard Theory"?

While I know that part of the pedagogy was giving students a grounding in dominant strains of formalism (in North American circles anyway), at times I felt a bit cheated out of a broader perspective.

Of course in practice many of the competing frameworks are weakly equivalent in terms of their descriptive power and the differences come down more to things like biological plausibility, computational complexity, how strongly you interpret the claims of UG, whether you have any interest in building tractable computational implementations, and so on. Or how you feel about the proliferation of empty functional categories, which was what bothered me the most from an empirical point of view.