There's some slight technical reason for it, but I think they swung a bit too far in the asshole direction with blocking too many.

The LTE rollout was completely botched from the start. LTE voice is technically supported on all LTE chipsets, but early on the voice spec changed. Early phones used LTE for data and 2G or 3G for voice.

Complicating matters further, AT&T and Verizon both have separate and slightly tweaked versions of the spec, as they didn't want to wait for it to be finalized, and of course they're both different in different ways. It's also why T-Mobile allows so many devices. They just rode their very fast for the time HSPA+ network until LTE was finalized, got generic hardware on the network, and flipped the power switch.

To top it off, AT&T was sued at one point for 911 not working due to a handset bug and they got very controlling at that point to avoid future lawsuits.

VoLTE is ostensibly VoIP over cellular data at its core. All phones have to talk with the correct SIP signaling on VoLTE for voice calls to work. With 2G and 3G, the circuit-switched method of signaling was much more standardized (although not necessarily simpler, WCDMA at its end spanned literal volumes of books.) This made it so phones and networks were more easily compatible for basic things like voice, 911, etc.

Now, on top of Verizon and AT&T thinking that rolling their own flavors of LTE was a good idea, every phone maker also had their own idea about how the VoLTE SIP signaling was supposed to work. Due to flaws in the LTE spec, carriers going rogue, and companies interpreting things wrong, it has turned quite literally into a clusterfuck.

TL;DR: It took a long time for LTE to standardize enough across product lines, and there are a whole bunch of phone models that don't talk the language quite right. So carriers chose to ban rather than make workarounds or work with the vendor to roll a software fix to the phone.