It's something that I wondered for years. The ship still moves, presumably it couldn't have huge operational costs. I just remember it been a really cool walk through ride and never actually found out why they closed it.
It wasn't that popular, (until it shut!), the inside (allegedly) was in need of much repair, and towards the end, it became a popular place for naughty pot smokers to indulge.
It also had issues of accessibility, one person of limited mobility could cause a fifty person conga chain.
I know, because my ex client started one!