I'm pretty sure it's the hardware. The SOCs they use in the mainline pi products are generally targeted at applications which aren't battery powered, so they do

I'm pretty sure it's the hardware. The SOCs they use in the mainline pi products are generally targeted at applications which aren't battery powered, so they don't focus on fast sleep support and similar power optimisations (which make the system design much more complex). Unfortunately if you want that you generally need to go with phone SOCs which are generally incredibly NDA-bound, very hard to buy even if you're as big as the rpi company, and have short availability windows which runs counter to requirements of large parts of the SBC market segment.