I understood that the rents kept rising which was making it hard to meet day to day running costs let alone make a living. Also some of the people moved on or passed away.
I think it was fairly swift - it had gone from being maybe ten stalls on a Saturday in the mid-1980s to maybe fifteen to eighteen stores when second hand started it's transformation to vintage/retro. It then seem to disappear over the space of one to two years in the late 90s early 2000s. That is a considerably fast exit but having hung around on the stalls, I can understand how easy it was not to sell much.
I don't think the impact of cheap clothing exports had quite been felt by that point and second hand markets are popular. Westmoreland Road market is now part of East Street market and still selling lots of crap!