Well, the Series X and S have an m.2 slot, as well. It's just taken up by the included hard drive. You could conceivably take that drive out, hook it up to a PC, clone it to a larger capacity drive and then put that larger drive back into the console. That's a lot of work, but it's probably doable.
The PS5 just has a second, empty m.2 slot into which you can stick a new drive alongside the old one. That's a whole lot easier than the process that I just described, but also more work than sticking an external drive into a rear expansion slot.
We don't know if Microsoft could've added a second m.2 slot. It's possible that they couldn't have even if they'd wanted to. For one, the motherboard may not support more than two PCIe-connected storage devices. After the internal SSD, it may've been a choice of adding a second internal connector or adding an external connector. Even if the chipset could maybe support more than two, the motherboard is so tiny that there may not have been room to. Of course, it's also possible that they didn't want to include an empty internal connector in order to dissuade people from opening up the console, and that would be unfortunate, but my point is that there may be technical reasons, too.
