Because they don't really complement each other: nuclear + renewable grid has basically the same holes as a pure nuclear or a pure renewable grid, nuclear doesn

Because they don't really complement each other: nuclear + renewable grid has basically the same holes as a pure nuclear or a pure renewable grid, nuclear doesn't like dealing with peaks in demand (you'd need to massively overbuild nuclear to supply the peaks as well), and renewables might deal with peaks in demand but the dips in supply are a problem (for which again, you'd need to massively overbuild). For both you want to pair them with either storage or flexible generation (and the most economical and scalable flexible generation is gas, the only renewable options are hydropower and geothermal, both of which are limited in capacity by geography). So you're going to be over-building capacity and building lots of storage whatever mix of wind/solar/nuclear you build, so why not build the cheapest (and faster) mix which tends to exclude nuclear? (now, you might in principle need less storage and overbuilding with nuclear than with renewables, but the difference has to somehow overcome the 3x-4x price premium on nuclear).