[99_00]

Because it was memorized.

Hot and humid climates result in (historically)rapid decay of plant and animal based writing surfaces

[anon291]

No that doesn't make sense. What would memorization have to do with millenia old preserved manuscripts?

Were the Vedas et al memorized before being written down? Yes, of course. However, regular discoveries are made of ancient manuscripts in various Buddhist temples.

[99_00]

What doesn’t make sense? That humidity affects the likelihood that an ancient text will be preserved?

Humidity doesn’t guarantee that something will rot. It makes it more likely.

The item will need to be protected from the natural environment. Or stored in a location that is an exception to the general climate.

Compare that to the Middle East. Where the natural climate assists in preservation.