The crowdsupply page has some insight on why they chose the SoM:

https://www.crowdsupply.com/mnt/reform

> Both SoC and memory chips sit on a System-on-Module (SoM) that we source from Boundary Devices, a US company that specializes in i.MX reference designs. We selected their "Nitrogen8M SOM" module for Reform because it is the only available module for which you can download the complete schematics and understand what every component does.