Why do we have to normalize the income of consumers when working with an Edgeworth Box in a simple trade model with Pareto optima?
If you by normalizing income mean normalizing one of the prices then yes you do not have to normalize it, but it makes everything easier since only the relative price matters. 
All price vectors (2 dimensions) that solves the problem will have the same ratio. There are therefore usually infinitely many price vectors that solves the problem.

Regards