There are several sites from where I looked for an answer to your question. You have already identified threats to this species, but your bigger question seems to be why the mountain beaver is so vulnerable when there are threats.

There are 3 main things that make the mountain beaver so vulnerable:

1.Multiple threats
2.Being a specialist
3.Rate of reproduction

1.The Point Arena mountain beaver is endangered, because of its sensitivity to its environment and has been threatened by many different problems. Some animals have fewer threats, and can cope with them in different ways. The mountain beaver seems to have too many to cope with at one time, making it particularly venerable.

2.The mountain beaver is a specialist. It needs to live in wet areas with a lot of cover and eat plants with a lot of water in them. Mountain beavers are a fairly primitive rodent and their kidneys are not very efficient. This may be why they need places that are wet. If their wet areas are drained, developed, invaded by dogs and cats, or plants they can't eat, they can't just move to a dry place and to live. Animals that are generalists can live in different types of habitats, so that if one type is destroyed, they can live in another. Generalists tend to do very well when humans take over a habitat. House mice are a good example; they'll move right into your house and start eating human food.

3.Rate of reproduction can effect the population of a threatened species. Only one litter is produced per year by a female. The females bear 2 or 3 young in late February or March. Other animals may have more than one litter per year, or more offspring, so there is more chance of rearing the young into adulthood, and a continuation of the next generation.

As humans develop more wild land, which species do you think will benefit? Which species will be hurt?

Check out theses web-sites for more information:

wikipedia mountain beaver

Endangered Species