From a developer and financial stand point, re-using as much as possible cuts down costs a lot. Especially if it still sells, it just means more profit for them. This compared to the costs of developing a completely new game, which would cost a lot more and may not do any better then one that had a much lower cost to produce. They're probably at the point where they aren't expecting to get any new fans for the series, so they're just producing them like clockwork not bothering to do much new with it. It's similar to what nintendo did, abondoning their fans to go for what makes more money for them- and only tossing something for the long time crowd here and there. I feel that they were trying some new stuff with BOJ, but in the end it didn't do well and it got shut down- which just cost hudson money. It was a complete minus for them because they refunded everyones money at the end, meaning that there wasn't a single cent made off of it.