The Miami Arena was a victim of timing. In the late 80s, arenas were being build much the same as they had been in the previous 30 years. When the deluge of new arena starting going up in the mid 90s with the large number of luxury suites and fan amenities, the MA paled in comparison. There were enough investors & political will to increase local taxes to build two new arenas, so they did. IMHO, south FL would have lost both the Heat and the Panthers if they would have continued to play in the MA after 1998.
BTW, while the American Airlines Arena isn't far from the MA, it is near Bayfront which makes it a bit nicer. I've gone to a few Heat games there but we usually end up parking in a shady area. The Bank Atlantic Center in Sunrise is way out near the swamps, but it's a pretty nice venue.