The biggest reason that October is such a popular month to visit Las Vegas is the Weather. It could not hardly be better. The average high is 82° F (28° C) and average low is 54° F (12° C). It is a drastic change to have such cool nights but that is what most tourist love.
Also, October is a very busy month for Sports Books, You are likely not to get a seat in the nicer ones all over Vegas, unless You plan well in advance. Football, both college and NFL are in the full meat of their seasons and Major League Baseball is in the playoffs. Every day there is something big happening to bet on.
And Octoberfest is in full tilt all over Vegas, a good excuse to drink beer and eat German Food.