Why don't other schools build a similar track? Doing that is going to cost a good bit of upfront money and you're not guaranteed you'll get the kind of business BU does. I'm not even going to try to count how many schools there are within an hours or so drive of BU that have track teams, let alone the number within what most consider a reasonable drive but it's really big. There are also loads of running clubs as well. Few locales have anything close to the potential track users that BU does. And in the event a school beyond driving range wants to send people a fast track with good competition to chase times flying to Boston is one of the easiest flights you can find.
Just as a semi extreme example, ask how many schools with track teams are within a couple hours' drive of Orono, Maine and how simple it is to fly there. Then ask yourself why a school chasing times would want to send a team there when it's easier and cheaper to go to BU?
Take this a bit further and imagine you're an out of school runner who isn't world or national class but is still competing seriously on the track. Maybe you're just looking to get your 3,000 under nine minutes or your 800 under two minutes. The odds are that you and most others like you live a lot closer to BU than to the U. of Maine, or lots of other schools. Indoor track is a really, really, big thing in Boston and its general region and as such there is a lot of demand for a good indoor track, so much so that New Balance decided it was worthwhile to build another one. But most universities are in places where it's unlikely they'd get enough return on their investment to justify building auch a facility.