Pan Am submitted a bid for the Bangor Sub when the MMA was being sold, but it was passed over in favor of a buyer who wanted to purchase the entire railroad (Fortress). And yes, I'm aware that CP has stated that part of the reason they bought the CMQ is for access to Searsport. Here's the thing: Searsport isn't all everyone is making it out to be. There's no direct rail access to the pier anymore (thanks State of Maine), the whole area would need to be completely rebuilt (at a minimum of tens of millions of dollars), there's enough of a NIMBY presence in the area to hinder it from taking off, and lastly, operationally, you can't run six axles down to Searsport, and most of the bridges still need work. Make no mistake, CP bought the CMQ for the (in)direct access to Saint John, despite what they may say publicly about Searsport.
The freight coming off NBSR is largely traffic that goes down to customers on Pan Am, it's not something CP can steal away by offering a better routing/rates. And right now is a slow time of the year for construction, so of course you're going to see less traffic, a large portion of it is lumber and drywall. Saying that the trains have rarely been over twenty cars is pretty disingenuous, considering that the trains are usually more in the ballpark of forty cars or so (I regularly see the train lists/brake slips from these trains, not relying on spotty facebook posts).
But the follow up, why wouldn't Pan Am want to purchase the Bangor Sub, considering it would cost less to purchase that than it would cost to rebuild the 60 miles of track east of NMJ (or even the 45 miles east of Old Town)? It would still provide direct access to Irving and CP, it would allow them to abandon the 45 miles of track east of Old Town outright, and the Bangor Sub is a decent 25 mph line, compared to the shaky 10 of the Freight Main east of Old Town. Furthermore, the small base of traffic that CMQ has would compliment Pan Am's current customer base around NMJ well. As far as CP in concerned, all the local traffic on the Bangor Sub is small time stuff, none of which they are too concerned about. They want the big time stuff, unit oil and potash to Saint John, and potential intermodal.
Finally, looking a little more at the long term, if Pan Am was trying to sell, what would be more tempting to potential buyer? A railroad with a mainline that is mostly 25 mph or better thanks to the purchase of the Bangor Sub, or the current one that has nearly a quarter of the mainline at 10 mph?