I believe a big reason for this is that the RP2040 is manufactured on a relatively modern process node. Mainstream MCU manufacturers use ancient nodes, which means using the same amount of SRAM is a lot more expensive area-wise.

It probably also helps that the RP2040 (and most Espressif chips too!) don't include any onboard flash. Adding a nontrivial amount of on-chip flash is quite expensive, so they just used that area budget for extra SRAM instead. If you want more than a few hundred K of flash you need to use an external chip anyways, so why bother with on-chip flash at all?