OK, I guess that was kind of long and rambling. It boils down to:
1. It's a pain, both in terms of what it provides and what it doesn't provide, compared to every alternative I've used
2. Despite being an ISO standard, it's a different pain everywhere
3. Even libraries which exist only to make it more consistent and less painful don't help beyond fairly trivial tasks on the 1 or 2 most popular free implementations
If any other language had this property, I would not use it; SQL is no different.
I think the problem is as you say... the SQL standard is too small and hence anything outside the standard (but actually useful) is implemented differently on every platform.
However, two points. First, the diffs aren't that huge (the underlying idea is generally the same, with some caveats, it's just the syntax that must be juggled). Second, most places I know usually pick one or two database systems and then make a very long commitment to them. There is no reason the developer shouldn't be able to pick up whatever SQL flavor his shop is using.
My original point is that any developer worth his salt should be able to pick up new languages fairly quickly, and since SQL is like a mini-language, it should be learnable in mini-fairly-quickly time.
1. It's a pain, both in terms of what it provides and what it doesn't provide, compared to every alternative I've used
2. Despite being an ISO standard, it's a different pain everywhere
3. Even libraries which exist only to make it more consistent and less painful don't help beyond fairly trivial tasks on the 1 or 2 most popular free implementations
If any other language had this property, I would not use it; SQL is no different.