Conventional commercial designs try to maximize kg-kms per dollar fuel cost or kg payload per dollar investment. Or minimize operating & maintenance cost.
All these goals need an economy of scale that forces few large complex turbines.
On the other hand, most solar planes focus on raw, dollar-agnostic performance / endurance metrics like max height or max range etc.
This is an entirely different optimization problem.
Also, with higher payloads of conventional designs the structural weight of a sturdy airframe is already a sunk cost. With solar planes your baseline airframe can be very slender so every strengthening is adding weight.
Another fundamental aspect is that in solar the generation is anyways distributed & not centralized. Electrical distribution is relatively easier vs mechanical transmission.
In a conventional design it is difficult to get distributed generation (many tiny turbines & even if you did the fuel distribution system would offset any advantages) & no easy way to transmit mechanical power to multiple props unless you add a mechanical-to-electrical generation system.
PS. A lot of this is speculation on my part based on your very interesting question. I could be wrong.