This. Instacart is really just a luxury service for the middle/upper class. Those same customers have Amazon Prime and guess what Prime Fresh will offer soon? (Whole Foods)
For starters it's a good bet that your average Walmart customer will pay approximately zero premium for home delivery. More broadly it's just not something people have voted with their wallets for. The Silicon Valley developer who deeply resents and is stressed out by a trip to the supermarket is just not your average consumer.