Part of the reason the Apple I is so rare, is that Apple offered an Apple I trade in program. Apple would destroy the boards of Apple Is that were traded in for Apple IIs.

* Not that there was really many to begin with.

What was the reasoning behind that?

It's because the Apple I had no built-in BASIC, and booted to a Monitor prompt. It was hard to use without a manual in front of you.

Meanwhile, the Apple II just let you put in a disk and boot a program. Huge difference in usability.

Probably to reduce support costs.

I recall my junior high school had only Apple IIs in 1995.