Two things that contribute to the downfall of brick and mortar stores are obviously, online venues that either have few costs or are nothing but drop-shippers. Second thing are the costs associated with stores. Rental, taxes (which in places like major cities can be $20,000/year. Plus utilities, etc. One of the worst things was that online sellers never even charged taxes until forced to start collecting it about five years ago. Also, around 1985, camera stores in New York and other big cities began selling telescopes other than crappy 60mm white refractors, they put pressure on telescope selling stores. More than a few of them have gone down as well.
That's absolutely true for all specialty retail, but for astronomy, I think there's the additional factor that with the growth of urban sprawl and light pollution, it's harder to simply see the stars, so fewer younger people are taking up the hobby.