Europe is the key. That's where the money is. You couldn't have a manager training on the job with one of those clubs. Villas-Boas at least won the EL before being offered Chelsea. And let's face it, when push came to shove against decent teams in Europe Moyes couldn't hack it.
In short, he gives bosses of CL involved clubs no reason to employ him.
For me thats a bit simplistic, its futile trying to compare Evertons European record to teams who can spend 100's of millions - because thats the deciding factor really. What is of relevance is getting a club from relegation fodder into CL/Europe anyway, its debateable which is the bigger quantam leap: getting a team that in the main had to wait untill the last of the season to snsure PL status in to Europe or being in Europe to actaully winning a European trophy.
Now that the word budget is being loosley applied to bigger clubs, i can see Daveys skills beocming more and more marketable.
I very much doubt hes been here ten years without job offers if im honest.