Because we already have a functioning fusion reactor.
Utility-scale PV now costs only $43/MWh. Investing in developing fusion reactors makes very little economic sense compared with capturing the output of the fusion reactor we already have.
The research should still be done, of course. It can have benefits to a future interstellar civilization - but until we're interstellar, PV is far, far more compelling.
Cheap intermittent sources are sufficient to destroy the economic case for expensive baseload sources. The latter have to be able to sell their output most of the time or else their economic case collapses entirely.
Utility-scale PV now costs only $43/MWh. Investing in developing fusion reactors makes very little economic sense compared with capturing the output of the fusion reactor we already have.
The research should still be done, of course. It can have benefits to a future interstellar civilization - but until we're interstellar, PV is far, far more compelling.