Cuz it's custodial. Wallet of Satoshi is a company that has the bitcoin and promises to facilitate your payments. But if they went bankrupt or if they were caused by the government to stop business, you would quickly understand that the money is not yours after all.

A wallet like Phoenix is non-custodial. If Phoenix were to disappear but you still had the app, then you would have everything you need to move the funds to another wallet without any help from Phoenix's company (Acinq).