This is the capitalist version of Vonnegut's "everybody wants to build and nobody wants to do maintenance." The kind of programming HFT firms want to do has vanishingly low diminishing returns basically everywhere else. They're used to lazily offering high pay and bad working conditions to self-driven people who learned C++ because it was the only tool available. Now other languages are mature and they are being caught flat-footed without a development pipeline for junior devs.
Strategically I understand - it was a pretty good bet that you could use the same "get gud" philosophy that still powers their trading desks, but they just got unlucky because we're in a moment where the net opportunity-cost for not-learning c++ is basically a reward (better work experience, often better money).