One possible reason I can think of is that phone camera apps will not proactively read PDF417 barcodes like they will QR codes, thus discouraging people from thinking they can scan and decode them.

That's may be a good reason.

My phone's default camera app can recognize QR and UPC (and certainly other things; but I have other tools that I usually use when actually-using barcodes so I'm not that familiar with this part of the camera app), but it doesn't seem willing to do anything with PDF417.

PDF417 has non-square pixels (or rather as it's called in barcode nomenclature "modules") which feels very janky - it was meant for linear scanners after all.

Oh, and quoting Wikipedia:

In practice, a PDF417 symbol takes about four times the area of a DataMatrix or QR Code.

Yes. They're clearly different things.

Which of these aspects offers a functional improvement in this application?

("Feels janky" doesn't quite cut the mustard, I don't think.)