So 3DS cartridges can store 8GB. Well, that's cool and all but how much would such a cartridge cost? Even if this is a possibility it doesn't mean that using such a large cartridge would be affordable. With cartridge systems in the past cart sizes could vary significantly on the same system. Usually larger carts became more common throughout the system's life. That's because the bigger carts were more expensive to produce. We even saw situations where bigger games, like some RPGs on the SNES, had a higher price then a typical game, thus passing the cost of the bigger cartridge to the consumer.
Companies want to maximize profits. If they can cut this or that corner and save a few bucks they will. With optical discs you can't really do that. A DVD is a DVD. They don't really make smaller DVDs that cost less to produce. There are dual-layer DVDs but if your game is smaller than the standard DVD (or CD back in the PS1 days), tough luck. You can't get corners. You've got 4.7 GB whether or not you need it. There is a cap on how much you can cut costs on the medium.
Carts don't have that limitation. Suddenly you can cut manufacturing costs by being cheap with the medium. You could quite easily have less than 4.7 GB. So why not compress that FMV further or, hell, get rid of it outright to decrease the size of the cartridge and save money? For years you would see games use a password because the publisher didn't want to splurge on a save battery for the cart. The model gives third parties excuses. They could fit a 4 GB game on a 3DS cart but maybe it costs a little more than a DVD. Or maybe they've just decided that they only want to spend X amount of money for 3DS games (because they can) so they'll never exceed 1GB. They've based their business model on a certain price range of cart, which may even be cheaper than a DVD, and so even though this DVD game CAN be ported to the 3DS, they don't want to spend the extra money, so it either doesn't happen or they compromise the port.
I don't think for a portable this is so important but for a console it is. They make the Wii 2 a cartridge console and third parties will just dick them around like always. It wouldn't even just be a problem of making sure the cost of the carts was comparible to discs. Providing the ability to offer cheaper carts with lower capacity is something that will undoubtably be abused. There is a lot of benefits in being similar to the competition. Same minimum, same maximum, same price structure - no one can **** you. It just would not be worth the effort.
The big advantage to carts is the lack of load times. Load times matter to Nintendo a lot, but we know that they just are not a big deal to everyone else. Most first party Cube and Wii games have such smooth and quick load times that they more or less don't have them at all. Yet on both systems encountering third party games with long load times is common. We know the hardware is capable of having short load times but third parties don't make use of it. That's because they know they don't have to. They know the consumer will put up with it. They know their game will still be playable, or even be considered a classic. They would have to push it really hard for people to not buy a game because of load times. The consumer doesn't really care and the developers don't really care. So the whole advantage is completely moot. So if the cost is just a little bit higher, kiss third parties goodbye. Saving money is worth more than no load times. If carts allow for cost-cutting methods that discs don't, then they will be abused at the expense of the gamers.
The key to third party support is being conventional. Being Mr. Oddball has gotten Nintendo jack **** for third party support for almost 15 years straight.