They have told that in a blog post as far as I remember.

The choise is probably because it is hard to get 5" this days? Ever heard of the world small volumes vs big volumes?

And since we talk about the HW:

For those who wonder about the CPU: We can't be sure it will be the same ARM core as the one used on devboard eg: iMX8M.

More realistic seems to be a iMX8M-Mini cause of the problems with iMX8M is eating to much power. There has been a blog post about the power problems last year. But my guess is people has forgotten it and people will soon whine about that also but hey is there something else we can choose? Just tell me... Maybe another CPU manufacturer? Maybe Allwinner LOL now try get techsheets from them in best case you get a google translate from chinese PDF. Qualcom? no way closed as heck. The one used in the RPI(Broadcom)? nope they want big volumes. TI naah to old CPU and also GFX closed.

So nope we are back on NXP land and iMX8 family or iMX6.

No someone will say there is a open competitor: Necuno mobile LOL. Open?? Nope something is really smoking ******** with their site and marketing.

The true is:

We should be happy Purism at least try make a open phone which is far from easy task. Indeed almost impossible. Many has failed already.

AFAIK librem5 is the most open hardware as far as I know. Not even rapberrypi can compare.