Power consumption. Maybe not so much on the Dev board, but IIRC I can get an ESP32 down to lower power consumption on a custom board.

So I might well start developing on an ESP32 board knowing that my final target is an ESP32 custom board.

It doesn't even have on chip wireless and the drivers are proprietary. Also external flash that can't be protected using fuses. If's fine for hobby level stuff, but that's about it. If you want to make a product and not have it cloned and the firmware copied, good luck. Also the ESP32 has a power management coprocessor allowing it to.operate on batteries, wake up and do stuff, then go to sleep.

The RP2040's highlight is the PIO and the ease of deployong the firmware, although I find flashing the ESP chips much more straightforward.

Anyway, there are still issues with the official 1.20 and 1.19 micropython esp32 spiram images throwing random guru meditation error messages from esp-idf. I'm using it them with several ESP32 WROVER modules, they all crash in the same ways. Maybe I waste 19€ plus shipping on a Nano ESP32 to see if it still crashes on the U-blox module.

Everyone is talking specs, which might be a reason someone picks a product over another, but I'll offer another anecdote:

The last time I bought some prototyping gear, there were a half-dozen options that met my technical criteria. The option we ended up choosing was the first place to answer the phone, discuss our specific needs, and give us a delivery date.

There's a lot of good prototyping boards out there, I'd bet a lot of buyers are making decisions based on how quickly they can get back to business, rather than taking a fine-tooth comb to a spec sheet.