Ok, my teacher back in high school explained that there were too choices. And to put it very simply, to break up the monotony of putting it after the infinitive all the time. Not a helpful answer, it'd be interesting to know the real reasons why.

Also, in European Portuguese the pronoun follows a verb. "Vejo-te," etc. If the utterance is negative or it begins with some kind of adverb, then you could say "não te vejo" or "ainda te vejo" - the clitic pronoun has to attach to something, basically.

In Brazilian Portuguse, however, the clitic pronoun is free to go before the verb like in other Romance languages as in "te vejo."