Why use l' and not en?

Question

If the expression is "faire exprès DE", why is the contraction "ne l'ont pas and not n'en ont pas'?

Accepted Answer

It is faire exprés de + infinitive. It isn't a noun that's following, which may be substituted by "en", it is an action, which is substituted by "le" (it).

Answer

Bonjour Olaedo,
To illustrate Chris's answer:

Il a fait exprès de casser le bol = he broke the bowl on purpose
-> Il l'a fait exprès = he did it on purpose (il fait exprès de quoi = de casser le bol -> verb)
 
Il a parlé de ton voyage = he spoke about your trip 
-> il en a parlé = he spoke about it (il a parlé de quoi = de ton voyage -> noun)

I hope this is helpful.
Bonne journée !