As far as I understand that "de" is used to give more information. And "de + le" = "du".
so why it's 'de taxi' not 'du taxi' ?
PS : I am not sure if I picked up the right question tags.
Une station de taxi is not dedicated to a single taxi, there are plenty of potential taxis that might use it so that can't be du taxi (i.e. de le taxi). If you are in a small village where there is a single station for a single taxi, you might say la station du taxi.
It is not either une station des taxis because station is preceded by an indefinite article, an indefinite station is unlikely to be the one of specific taxis, however, la station des taxis is technically possible in the latter case, e.g. la station des taxis de banlieue.
Finally, as you are talking about a station dedicated to taxis, the preposition de is used. It introduces a relationship between a concrete noun (station) and a generic collective (taxi). This construction uses no article (a.k.a. article zéro), thus une station de taxi.