I don’t think we have data on this, but for starters they make 20% more from every sale. Being on DMsG therefore needs to result in higher sales compared to DTRPG to be worth it.
Whether it is probably depends largely on whether you can do a similar product without their IP, as I doubt the DMsG gives you that extra visibility to get you those additional sales.
So if you want to make a D&D adventure, make it generic instead of setting it in Phandalin or wherever. If you make supplements specifically for a WotC adventure or setting, then you have no other option than DMsG however.
And my thought is that I know there is some segment of the supplement/adventure buying public that wants stuff they can put directly into their Forgotten Realms, Ravenloft, Dragonlance, etc campaign, and are not necessarily interested in putting in the work to take something generic and use that instead. It doesn't matter if Strahd Von Zarovich is Dracula with the serial numbers filed off, or how easy using another product in an existing D&D setting is - some people just won't do it.
There are designers who have sold their stuff on multiple platforms including stuff they've written separately for DMsGuild, and based on the dates they were added to the catalog, they were writing for products for both. My assumption is that there has to be some value for these designers to write stuff for the DMsGuild. I don't know if that's a difference between they write early products for DMsGuild for the name recognition, and then migrate away when they can sustain completely independent products under their own banner or what.