A backplane is nontrivial to design electrically/mechanically and expensive to manufacture (even just some high quality connectors will quickly run up your BOM). If you also want to carry networking there you'll end up having to do some semi-custom network switch design, which locks you into a particular switch/ASIC vendor 'forever', or at least vastly increases friction when wanting to upgrade. Yes, they already have their own PCBA and some mechanical design, but it's very simple compared to what it takes to design a reliable backplane with an integrated power supply and network switch.

At the end of the day this a low-cost system for low-cost devices. IMO the little benefit from having a backplane is not worth the R&D cost and the downsides of fully backplaned blade systems.

And this is not just about this project: from what I see, the industry seems to have rejected fully integrated blade systems. Dell's M1000e is dying, and I don't think I've seen HPE bladecenters in years. Instead, semi-integrated systems like Supermicro's high-density offering is king. No proprietary chassis management system, no proprietary network switches, no locking yourself into whatever the backplane can carry.