Why have the subjunctive and indicative converged in Modern English?

I mentioned this in another question, but just because the morphological inflection is disappearing, that doesn't mean the subjunctive mood is actually disappearing from the language. Just like when most of our verbal inflection disappeared (now it's "I go", "you go", "we go", "they go"), that doesn't mean we lost verbs for first person singular and plural, 2nd person, and so on.

Nohat's short answer gives the main point — if 9 out of 10 times the form looks identical to a MUCH more common form, then over time things might converge and regularize. A linguist would call this "paradigm leveling". Less common words and structures tend to regularize faster than more common ones (which is why words like "to be" and "to have" are irregular in so many different languages). The subjunctive is rare and not that distinct in English, so it is in trouble.

We see the past subjunctive form only in "to be", but we see the present subjunctive in the third person singular form of any verb — it has no "s" at the end like the indicative form.

So, this is of course subjunctive:

If I were ten years younger... (often said "If I was...")

But this is subjunctive too:

So be it.

It's important that he arrive on time tomorrow.

There are a bunch of examples here that include this other kind of subjunctive. This is just anecdotal but I haven't noticed this one disappearing as much.

Is this a general trend in related languages? Well, in Swedish this seems to be happening. In German one subjunctive form is used all over the place, and the other is used pretty much just in newspapers and journalism in general, but it is at no risk of dying out (it has legal implications akin to those that make the word "allegedly" so important in English journalism).

Part of the answer is that the subjunctive ended up with almost no functional load. In most of its uses it is distinguished syntactically from other constructions, so morphological distinction is redundant. (e.g. 'Long live the King!', 'I demand that he be silenced' The major exception to this is in conditionals, where it traditionally distinguishes irrealis (counter-factual) from realis conditions:

• If I was there, I don't remember. [Realis]
• If I were there, I would give him a piece of my mind. [Irrealis]

But even there, realis antecedents are relatively infrequent, and in any case it appears that English speakers are often unaware of the distinction, though it is alive and well in some languages.

So the only case in which the choice of subjunctive verb alone makes a difference to the meaning is one where the difference is not now widely appreciated.

I think the short answer is the subjunctive became marginal when the forms distinguishing subjunctive from indicative became identical, leaving only some forms of the verb be to mark the subjunctive.

In short, The English verb system was different in Old and Middle English, but it had basically syncretized by Early Modern English; thus the subjunctive is still there and very alive in English, particularly literary English, but it's very inappreciable, which can make it difficult to see:

If you be good and if you eat all of your vegetables, I will get you some ice cream.

If you were good and if you ate all of your vegetables, I would get you some ice cream.

The President has demanded that we cease our action.

My only goal is that I win that race tomorrow.

It's best if he stay home today.

If I had known, I would have said something.

If you spoke English as well as I, the subjunctive would be child's play.

God strike me dead if I be lying to you about the subjunctive.

Whether I be brash or demure, the subjunctive shall live!

God bless the subjunctive and God forbid the subjunctive ever die!

My only wish for Christmas is that my father be here to celebrate it with us.

If there be gold, I shall find it.

If there were gold, I should find it.

I pray that he see the error of his ways.

We can only hope that he find true love.

It is very important that you not speak ill of the queen.

If I were President, I would cut taxes, unless I were Republican. (lol)

These are just many examples and, as one can plainly see, the subjunctive doesn't look any different than the indicative most of the time. And though this be the case, it's still considered the mark of the educated speaker. If I were you, I would learn it.

Modern English is syncretic and other languages like French are inflectional. Syncretic languages don't inflect their verbs too often so it's hard to see the subjunctive. It still exists though and is still said.

"I wish it were not the case." "I demand that he do it."

It's a pretty easy rule once you learn it.