One of the great mysteries with MF in my view is the low base ISO, but I guess things add up. No microlenses, perhaps more othogonal CGAs? Pixel size should be ISO neutral, as FWC is proportional to pixel area, but so is the number of incident photons.
"No microlenses, perhaps more othogonal CGAs" - together these add up to lower net quantum efficiency. Yes, that's a factor alright. One notices that the relatively few MFD cameras/backs with microlensed sensors have ~1 stop higher base ISO than their stablemates of the same pixel tech and size but without microlenses. P21+ vs P20+ and P25+; P30+ vs P45+; Hassy 31 vs 39 MP & 40 vs 50 MP...to take a few examples.
"Pixel size should be ISO neutral, as FWC is proportional to pixel area" - that's true in general, but don't forget the 3rd dimension - physical pixel junction depth into the wafer - and the "4th dimension" - the energy barrier set by the anode voltages, which control how densely one can pack the photo-electrons and still hold them in the "energy well". Like charges repel each other of course, so if you want to squeeze tens of thousands of negative electrons into a smaller volume, you need a stronger positive anode voltage centred on the pixel to overcome that repulsion.
So by exploiting at least one of these additional dimensions (I honestly have not checked which), Dalsa have often delivered greater FWCs for a given pixel area than Kodak. The other factor is that Kodak have pretty consistently had chips with higher spectral transmission efficiency than Dalsa. So the Kodaks have often had smaller pixel capacities AND they filled them at a faster rate: two things which raise the base ISO. This explains why hardly any Kodak-based MFD unit has had a base ISO of 25***, but several Dalsa ones have had...and when the Dalsa ones have started at ISO 50 or 80, the equivalent Kodak ones have started at ISO 100.
*** AFAIK, only two Kodak-based MFD units have had a base ISO of 25: the Sinarbacks 44M and 54M. But I've never understood how, since all the other MFD manufacturers using these same two sensors have set their base ISO to 50 or even 100. Perhaps as tethered backs aimed at studio use, Sinar were setting them up for more extreme ETTR than would be safe under outdoor light conditions.
This highlights another factor which muddies the waters: manufacturers have some leeway to choose what they like for base ISO. Thus, to take a recent example, Phase One set the Dalsa 40MP sensor to ISO 50 in the P40+, while Leaf set it to ISO 80 in the Aptus-II 8. There can even be inconsistency for the same manufacturer: Leaf then went on to set the same sensor back to ISO 50 in the Credo 40. So perhaps one shouldn't read too much into the base ISO value, since deliberate variations of up to 1-stop for the same sensor are common.