The hardware is much more sophisticated, miniaturized, and high-speed, and the supply chain is much more generic; a Ledger is a really primitive piece of hardware with a simple display interface and a ton of empty space inside it. An iPhone has no wasted space and extremely high-speed onboard buses that would be hard to sniff. Additionally, with Ledger, you know that every customer is going to be using the device to access crypto, so it's an easy target nearer the head of the supply chain. With an iPhone, you have no idea what end-user to target the implant against.