Yeah 9-11 basically killed the cross border community in eastern NB. I grew up in Richmond Corner, and in the 80s/90s, going over to Houlton was as common as going into Town (Woodstock). Houlton's Walmart was basically built because of all the NB visitors, and a run down to Bangor was as common as a run down to Moncton or Halifax (if not more common).
After 9/11 and the border tightened up, those visits died off a lot (my parents still live there). Plus, the NB retail landscape improved significantly as retail chains started noticing the Maritimes. Walmart, Tims, McDs, KFC, Pizza Hut, etc... all came to Woodstock. Costco came to Fredericton, and Moncton just became a shopping hub in general. So with tighter border restrictions, a worsening exchange rate, better highway connections in NB and better shopping options in province, all led to the crossborder shopping habit basically drying up.
As for why I-95 went up to Houlton? I have no idea. I suspect wanting to spur development in Northern Maine was a factor. Also, remember Fort Fairfield used to be a major US Military base back in the day, so having relatively close Interstate Access to it was probably a boon.
The NB-95 connection is only relatively (15-20 years old) new; happening when the TCH was twinned. I remembered when the TCH was being twinned, I suggested to my dad that they would finish twinning to the border, and he scoffed at the idea, saying it would never happen because it wasn't worth it. So I was rather surprised to see them do it in the end. It's a nice stretch of highway and useful for me going home, but I'm still a little peeved that they closed off access to Plymouth Road, forcing me to go all the way up to Richmond Corner.