Bitcoin is limited. Therefore more people using it = higher price per bitcoin. This is exactly why the price will go up (by enormous factors) in the long term, regardless of any temporary spikes and drops (which are just incidental market hype and panic movements).
Why would more people use bitcoin?
Because of the many great advantages, like the points listed
here or
here.
Similarly, people would ask in the early 90's:
"Why would more people use email, when regular post works just fine? Email is just cyber stuff for nerds and techies."