Not very surprising. When it comes to formal verification, you get the biggest bang for the buck (by far) via focusing on what the nLab wiki calls 'synthetic' mathematics, viz. fairly self-contained subfields where the 'rules of the game' may be somewhat complex in their own terms, but can be stated without relying on a massive amount of prereqs. 'Fashionable' math tends to be just the opposite: easy, logically-simple entailments, but building on very complex prereqs.
It's obvious why formalizing the latter is comparatively hard: you need to work on the prerequisites first, since your formalization won't be usable without them! Also, since the formalized-math field is still quite fragmented, large projects (such as formalizing a big chunk of some basic curriculum) are discouraged to an even greater extent - quite simply, it can't be assumed that others will be building upon that work.
It's obvious why formalizing the latter is comparatively hard: you need to work on the prerequisites first, since your formalization won't be usable without them! Also, since the formalized-math field is still quite fragmented, large projects (such as formalizing a big chunk of some basic curriculum) are discouraged to an even greater extent - quite simply, it can't be assumed that others will be building upon that work.