Yep. That's the big one - it's super fucking easy to get from Connecticut to Boston. You can take a car or mass transit.
Connecticut is such a small state that most of the Suns' natural fanbase is probably already centered in Boston. There's a reason UConn is playing BYU in Boston this season - it's naturally where most of their alumni probably are (besides NY) due to proximity. And even Suns fans who may live in NYC can get to Boston easily.
I totally understand why the league put the Suns in Connecticut to start back in 2003 after moving from Orlando - UConn was on the rise as a power in WBB, having won titles under Geno in 1995, 2000, 2002 and would go on to win in 2003 and 2004. Very similar to LOVB putting teams in Austin, Madison and Lincoln - big volleyball fan bases via college. Same with the likelihood that AUSL, once they decide where to put their teams, will probably put one in OKC and one in Alabama.
The league has blown up in popularity though, so you don't have to keep the Suns in Uncasville, when Boston is 100 miles away - bigger market wins the day.