There is an interesting article in Wikipedia (http://en.wikipedia.org/wiki/English_subjunctive), where you can see that it hasn't always been so. For example:
In Early Modern English, the past subjunctive was distinguishable from the past indicative not only in the verb to be (as in Modern English), but also in the informal second-person singular of all verbs. For example: indicative thou sattest, but subjunctive thou sat.
As to your actual question, languages evolve in ways not easily understood, and certainly not following other languages. When a certain form doesn't serve the purposes of communication, then it slowly dies away since speakers avoid it and replace it with more functional forms.