For Ren'Py, I find that it usually relates to the desire to keep the file size under a certain amount, due to limitations in free web-hosting. For example, if you have a free membership to MEGA, you will only be allowed to download 5gb per day, meaning that a game over 5gb would take multiple days to download. Also, just generally the time it takes to download. That's no fun.
For RPGM, you have events placed on a map. Those events may have many "pages" that progress sequentially. If you keep putting more and more events with more and more pages onto the map, the engine can start to lag, and design work can simply become cluttered and cumbersome. There may also be times where some fundamental change is made that would otherwise force people to start from scratch (i.e. "Old Saves Won't Work"), in either program. Making a new game can allow you to clear out old events on a "fresh" map, or avoid forcing players to start over.