Because in free America configuration it's not legal to transfer it to a MA or CT resident, even in NH or VT. It would need to be modified before you could take possession of it.
In some of the free states there are gun shops that will sell to MA residents and do the required modifications before selling you the rifle.
Stateline in Mason NH is one of them - I've bought a number of rifles from them and they did the mods before I picked up the rifle.
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ooo, I see they are making 5.45 conversion kits for it too!
Linky?