For the same reason PHP gets so much hate - both are tools with valid uses, but because they make hard things easier for people, that means that people who have no idea what they're doing can do very bad things.
Hating on a tool is ridiculous.
Hating on a tool is ridiculous.
This is not at all analogous to PHP where there is an abundance of alternatives to it's mediocre design. PHP isn't hated because of its concept - a "web-based scripting language". It's hated because it's done very poorly. Just like a lot of bad ORMs Fowler refers to.
"The short development cycles needed to stay ahead of the competition demand a web-centric scripting language that is easy to maintain and update."
http://public.yahoo.com/bfrance/radwin/talks/yahoo-phpcon200...
They didn't choose Python, or Ruby, or Java, because none of those languages are as expressive or shut-up-and-get-out-of-my-way as PHP. Just because a high schooler can write PHP doesn't mean it's a toy not also meant for world class engineers.
However there are plenty of bad ideas in the language itself.
ORM is bad because the impedance mismatch between an object and a relational model is very difficult to get right.
Frequently, for example, one iterates over collections of objects in the OO paradigm. A naively written ORM, or one without sufficient introspection into the loop intent, can translate that into N select statements, each incurring a network round trip.
The end result is that OO programmers have to understand not only their object code, but also the relational model, and finally too the ORM's peculiarities and suboptimalities. So in attempting to solve one problem, most of the time you end up with three.
That said, there is a sweet spot in ORMs for simple code (e.g. most web apps, where ActiveRecord and Sequel and the like are fine). But there's no sweet spot for PHP code. Every line written is technical debt.