I don't think they wanted an M4 per se, but instead wanted a bluetooth radio to save on the cost of a screen. The Nordic chip just happens to be based around an M4.
IIRC Nordic has a M0 version with bluetooth radio--the BBC micro:bit and a few other maker boards used it. But perhaps it's not really available in cheap quantities anymore and a way overpowered M4 is what they got.
Yeah, those are the nrf51x line, this is the nrf52x line. It seems from the outside at least that Nordic is pushing the nrf52x hard even in price conscious designs. Even the micro::bit was respun for a v2 with a nrf52x also (including it's M4). Albeit I don't have much direct data on that.
Yes, the 51 is really old by now. The 52 line extends down as far as the tiny nRF52805.
The 51822 is not nearly as fun to work with as the 52s, e.g. current Apache Mynewt with NimBLE doesn't even fit into the 51's tiny flash, the 51 doesn't have systick, and so on.