jatlh SuStel:

-wI' has the effect of nominalizing the verb into the subject of that verb. A vutwI' is the thing that performs vut, the subject of vut.

If you were to say *vutlu'wI', you'd be trying to nominalize the verb into a nonexistent subject. No one in particular performs vutlu', so it makes no sense to talk about the noun that performs vutlu'.

As I said, I'd like to hear what MO has to say on the subject.

I'd like to hear what Okrand has to say on every subject we ever question. That's not an argument against what I said.

It's bound to be more nuanced, not to mention a lot less bombastic and opinionated.

-- 
SuStel
http://trimboli.name