Or maybe it's due to iOS making up only 27% of the whole marketplace where Android makes up 71% of it. Why waste your time making malware for a system that only has a market share of 27%? It really isn't more "Secure" it just isn't worth your time.
That is simply because most Android users are poor and buys dirt cheap phones. Pretty much 75% of all Android sales consists of sub 200 dollar phones (with >>barely<< 1 year of software support).
There's no money in that market so Apple does not want it. It is as simple as that.
Look at App Store vs Play Store sales, and you will see that Apple earns waaaay more money than Google even with 1/4 of the market

Because again, most Android users don't have money to buy apps, they only use free apps riddled with ads and they pretty much all have bad optimization and performance because they have to support 1000+ different android phones and configs. Yeah I know, my work phone is Android and what a mess it is.. Terrible optimization across the board, only top100 apps gets decent optimization however not even close to the optimization iOS apps get.
This is the reason why iPhone feels much faster in real world usage. Superior SoCs too. For years Apple have had the most advanced SoCs, easily beating top android phones in real world usage.
Better chips and better app optimization = Win
Whole Apple ecosystem is just superior compared to Android which is a mess. Always have been. Always will be. Tons of fragmentation. Lack of support etc.
iPhone gets 5+ years on average.
Most Android phones barely get 2 year of support and this is mid to high-end products.
Cheap androids barely get 1 year of proper support and bugs on launch will most of the time never be fixed. The companies don't even bother. They will rather sell you a new dirt cheap phone the following year

Sad but true.
Only Google with Pixel line have stepped up for real when it comes to software support, sadly they always have tons of bugs and other issues..