Why did the achatinella mustelina become extinct?

Habitat destruction caused by humans and therefore increased competition with other animals for food caused the achatinella mustelina to become extinct.

The above answer is not entirely correct.

The main reason for the extinctions of not only Achatinella mustelina (if it is indeed extinct) and a large percentage of other native Hawaiian land snails is:

1.)the accidental introduction of rats to the Islands and, more critically..

2.)the deliberate introduction (as a biocontrol for the Giant African Snail, a non-native agricultural pest) in the 1950's of the Rosy Wolf Snail or Euglandina rosea, a cannibal snail which is a voracious predator of Hawaiian land snails.

This, combined with very low reproductive rates (many Achatinellidae give birth only once or twice a year, to single keiki) has proven disastrous for Hawai'i's native arboreal and land snails. In giving birth, the parent snail holds the egg inside its shell and when the baby crawls out, it is a fully formed miniature of the adult. Some (myself included) would liken this to live birth, which gives these snails a humanlike quality.

It is indeed a tragedy that many of these natural wonders (which occupied Hawai'i for millenia before the coming of humans) are disappearing within the span of one human lifetime.

Many of Hawai'i's native forest birds have shared a similar fate.