Mistwell
Crusty Old Meatwad
I would guess it’s because of the pact boons being invocations in this packet. Because some invocations now rely on having other invocations as prerequisites, so if you took pact of the blade at 1st level, then took thirsting blade at 5th level, they don’t want you swapping out pact of the blade at 6th and being left with an invocation you no longer meet the prerequisite for.
You know, that sounds like the likely explanation. Thanks!