Why is Beaumont know as boomtown

Due to the fact a massive oil well had been discovered there, which lead to Spindletop and other oil companies coming to drill for the valuable oil.

Explanation:

Oil at the time was quite valuable due to it's many purposes, hence, once oil was discovered, many prospectors and companies came to get rich off the oil. Those who were looking for work could also get jobs on the rigs positioned there.