It's operationally easiest to have it nearest to Reading Station so the turnbacks can be staged quickly around adjoining thru-Haverhill slots and occasional Downeasters that get as-needed re-routed to the Reading Line. They're basically shooting for an identical setup to what exists with the crossovers 1/2 mile north of Haverhill station, where there's a designated engine holdpoint for reversing between the outbound-side platform at the end of a run to the inbound-side platform at the start of a run and such switches are frequently done around Downeaster and freight slots that scoot around very close to the T turnarounds. 93 is 2 miles from Reading Station and there'd be a fair amount of single-track to traverse for reaching the engine holdpoint, so the meets under Regional Rail would be a little more brittle.
I mean, if the community opposition is fierce enough they could do it just to get what they need while quieting down the rabble. But it would be less flexible than having one adjacent to Reading Station.