It got buried because you are demanding a dev comply with your extremely stupid request. They expanded it for a reason, sorry you have an old3DS that requires additional resources to function for the mods. Maybe you should try asking in the thread why they felt the need to run the mod pack in extended memory mode on the 3DS instead of being rude and thinking you know better
https://gbatemp.net/threads/mario-kart-7-custom-track-grand-prix-7-ctgp-7.396936/
But to answer your question before you go annoying them in the thread. The reason for the expanded memory mode is that this mod pack needs additional resources to function. When loading mods, cheats, and anything else external into a game, those will be loaded in RAM as well. In small cases like cheats or one or two mods, they will most likely only need a few additional resources but not more than the stock resources used by the game. For example, removing outlines from Pokemon games isn't going to be that labor-intensive on the resources. But when loading large amounts of additional mods, like the CTGP-7 mod pack, more resources are going to be needed. The 3DS stock amount of resources used by Mario Kart is very low, thus they need to tap into expanded memory mode to load in the entire mod pack. Without expanded memory either the pack crashes completely or lags/glitches. Simply put, if you want the mod pack to run then you need to be willing to allocate the resources for it to run. Either that or make a gimped version of the mod pack that doesn't require expanded memory mode.