If you have access to Le bon usage from a recent edition, I would suggest you look at §1121a.
It basically states that the French language is not satisfied to put two propositions side-by-side without any visible/audible separation between them. In many cases, as here, the separation is incarnated by the addition of que.
À peine avait-il son bonheur entre les mains qu’on voulait le lui reprendre. –Gustave Flaubert, L’éducation sentimentale
The negation in your example also seems to play some role, since the positive version would sound more natural with et or mais, while either of the three (que, et, mais) could be expected for the negative version:
Je suis dans cette ville depuis dix minutes et je rameute déjà les foules ! (à cause de ma popularité !)