Todd's answer's a good one.
A more personal answer is that most of us who've done the work of porting the ACS to Postgres did so for one reason: so we can use it.
If someone's language bigotry is strong enough for them to want to port it to PHP, more power to them. No one's stopping them. But it's not on our agenda because OpenACS, as it stands, does what we want it to do - let us use the ACS on an Open Source database.
If our goal was to win popularity contests by providing an Open Source toolkit, we'd probably do something like PHP+MySQL. But we're not into popularity, we're into providing tools we can use with the least amount of effort we can get away with.
