One of these days Hopkins is going to realize that playing teams = dressing on gameday. Dressing on gameday = "hey xx grab your helmet and get in there" moments. If you want to play as a young unproven then special teams is the path.
I assume a lot of that is the fact that Higbee has really solidified himself as the pass catching TE, and we haven't dipped into our "two receiving TEs" portion of the playbook/heavy 12 personnel looks. Mundt is almost certainly the better blocker between the three TE's behind Higbee, and Higbee is the most consistent pass catcher/route runner of the group. Mundt is limited in his pass catching abilities though so I think they would like one of the young guys to step up for that role eventually. I believe Jourdan said that Hopkins spent time in the offseason bulking up a bit and working on his blocking so they probably envision him for that spot next year.
Dressing Harris or Hopkins is probably just not necessary at this point, but whenever that day does happen, barring any health concerns, it'll probably be an indication of a more run heavy/12 personnel gameplan vs the pass heavy stuff we've been rolling with lately.
Running back concerns could also be factoring in to who they're dressing in regards to the TE's.
A final note, if our ST is doing alright without either of the bottom two TE's dressing, why risk any injury to them? We're pretty light at the position after Higbee, at least in terms of established ability, if Harris or Hopkins were to get dinged up on ST that would probably make the coaches sweat a little regarding the health of the top guys.