I have iPhone 6 plus. Why can’t I download ios 13?

You can’t. It lacks the hardware to update.

Iphone 6 does not have enough RAM to run any iOS version past iOS 12.

I have an iPhone 6 Plus. How can I download iOS 13?

Try reading this thread

It doesn't matter what the app requires, you phone doesn't have the hardware to upgrade. If you need iOS 13 or higher, you need to buy a newer phone.

I want to download iOS 13

Then buy a newer phone that can run a newer OS. Your 6 plus can't run anything higher than 12.5.5.

P.S. iOS 13 isn't available anymore, you're options on a newer phone are 14.8.1 or 15.1.