DD and BD both have a more interactive environment than most RPGs, which the games need to keep track of. If you move chairs around or drop loot, etc they will remain that way when you return. There may be other factors, but the additional bookkeeping the games do (environment, NPC status and location, object status and location) means there is additional cleanup when the game exits.
Other than that, your guess is as good as mine (except that the games were done in C++ and not VB).
