There is no single reason that will push me and many other people to use Proton Wallet instead of Electrum and Sparrow.
Me neither, to be honest. I am using a combination of hardware wallets, desktop, and mobile wallets, and see no reason to bring a web wallet into the mix. Regardless of the company behind it and its non-custodial nature, it remains a web wallet that relies heavily on storing sensitive data on centralized servers. A leak, hack, or an unhappy employee could cause a lot of problems.