While you’ve got lots of answers, one that’s been left out is the slower production of “even isotope” plutonium.

Only about 2/3 of U-235 neutron absorption creates fission. The other 1/3 simply does nothing besides creating U-236. Ultimately, U-236 through a series of decay and more adsorption reactions becomes plutonium 238 (or 240) which can’t be separated from the Pu-239 that is used in nuclear weapons (basically, we can’t “enrich” plutonium like we can uranium.)

So, for a given amount of Pu-239 produced, the RBMK with lower initial enrichment has a “cleaner” material.

This is a gross simplification and there are other things that can affect this, but it’s part of the equation.