GrubHub partnered with Amazon. Amazon Prime now includes a GrubHub+ account.
I have Prime, but have yet to use GH+, but that's more because I just stopped getting food delivered in general. Not because of the price (I used DD and had the DashPass which is very much worth it if you get at least 2 deliveries/month), but because I started getting deliveries with missing items. When that happens, they won't offer to do another delivery, they'll just either refund the item or I can go pick it up. Both options defeat the purpose of ordering delivery. I either don't get my food, or I have to leave the house.
Doordash prioritized selection, they did online ordering even when restaurants didn't want it. Eventually the demand was high enough that restaurants adapted. Grubhub focused on the traditional take out restaurants. Doordash also mixed in multi-delivery to handle mixed alcohol/food purchases.
Beyond this, tech always follows a power law distribution in returns. Doordash is the number one player, and as such will be about 10x bigger than the next runner up. How much of this is network effects/first preference vs. manufactured return is anyones guess, but most people do not want to spend time price matching doordash/grubhub on orders - so whoever has the bigger network/more customers will get the most folks.