The simplest answer is, this is our first phone, please bear with us. We’re working on some amazing things but it’s hard to do it all at once 
More to the point though is we have to have a baseband (cellular chip that talks to the cell network) that we can seperate from the CPU. This is to allow for a killswitch and that the baseband does not have Direct memory access (DMA). See Kyle’s post about killswitches on the Librem 5 and Nicole’s post about our choice in using a seperated baseband.