It isn't actually the subjunctive. People often call the "were" of "I wish I were" subjunctive, but that term is much better used (as in The Cambridge Grammar of the English Language) for the construction with "be" seen in "I demand that it be done." The "were" form is often wrongly called a past subjunctive, but of course "it were done" is not a past tense of "it be done". The difference between the two is that the subjunctive construction occurs with any verb: "I demand that this cease" is a subjunctive (notice "this cease", not "this ceases"). The relic form in "I were" is only available for "be". For all other verbs you use the preterite: "I wish I went to New York more often." The Cambridge Grammar calls the "were" form the irrealis form. It is surviving robustly in expressions like "if I were you", but even there it has a universally accepted alternate "if I was you", and there is no semantic distinction there to preserve.