Because they are still actively manufacturing HR54s. Maybe they aren't quite ready to go all-in on the HS17, but want to wait until the HS27 which probably fixes some first gen issues with the HS17. That they won't let you use other hardware with the HS17 may figure into that as well.
One thing I found interesting in that permissive change doc is the swap of BCM45208 for BCM45308 tuner chips. Maybe they switched because Broadcom said they were discontinuing the 45208 (I don't know if they are, just guessing that could be a reason - FWIW Broadcom's web site lists it as "active") but the 45308 adds DVB-S2X capability which could also be the reason. I've long wondered how (and WHY) Directv would implement bonded transponders without DVB-S2X which natively supports it - they'd have to handle that part in software, which seems like an unnecessary burden. Given when the HR54 was designed they may not have had any choice but software bonding for early (but limited) 4K support. Which makes me wonder if perhaps the reason why the HS17 is limited to only supporting two 4K "tuners"/TVs has to do with limitations of handling transponder bonding in software.
If so, an HS27 (and theoretically a 'rev 2' HS17 if it is easy to tell them apart via part numbers) using BCM45308 chips might support 4K on all 7 "tuners"/TVs. While some have claimed Directv will be bonding three transponders because of the goofy way they appear to be assigned, I still believe they will stick with their original plan of two. That makes a future HS27 supporting 7 4K TVs fit perfectly with the DSWM30. While I don't know for certain, I'm willing to bet the DSWM30 relied on Maxlinear's second gen DSWM chip, which went from supporting 24 tuners to 32 - that way the DSWM30 only needs one DSWM chip to support the 30 tuners across its two outputs, and explains why when we have 21 tuner LNBs we didn't get a DSWM42 instead. They wouldn't have done that if a Genie supporting 7 4K tuners would need 21 SWM channels.