"Why is that?"

Good question. I wasn't sure myself. I seem to recall RJBS saying that initializers were his first ever contribution to Moose, but that he regretted ever adding them.

The best reasons I could come up with were:

• It leads to inconsistent behaviour when an attribute value is set via the constructor versus a writer method. (Of course, this isn't an issue for read-only attributes.)

• Initializers can't be inlined whereas type coercions can be partly compiled into a more efficient form, and have potential for improvement.

Having asked in #moose, I've had the following additional suggestions:

• The initializer isn't called until after type constraints have been checked, so initializers don't work for one of the main use cases you'd expect them to be useful for: to munge values to conform to their type constraints.

• (I'll add to this list when I receive more responses from IRC.)