Why does gas form a star instead of a black hole?
Essentially gas does, it just happens to form a star first. 

Mass is not the only factor in creating a black hole. You also need for this mass to reach a high density. In the process of doing this, a star usually forms. The energy producing processes in the star interior produce a pressure that balances the gravitation attraction. This prevents a star from reaching a critical density needed for black hole formation. As these energy producing processes run out of useable fuel the star will eventually collapse onto itself creating a black hole. 

So you can't just take a large volume of gas and create a black hole. Other physical processes occur.