For the most part, it comes down to UM availability + costs. Most of the time the -500 has older models available for dirt cheap in the UM.
In addition, many airlines do favor saving the initial million (or the 5k on the lease) over sometimes unneeded boosts. Unless you’re flying into the Scottish Isles or Southeast Asian islands, you don’t need runway requirements less than 1000m. Even YTZ (by far the airport that bugs most people out w/ runway requirements) and EYW will handle the -500 just fine as will almost all Philippine gateways (speaking from experience on that one). And range, of course, is very subjective, but most ATR routes fit the -500 as well.