Mmmmmmm ....
Always the same type of question ... the answer is also because when you go into a twin the level of systems complexity of the aircraft is increased, the tasks and envoironment of operations more complex and unless you manage to get a job where you can be mentored (say by being a co-pilot) the possibility of f**king it up if something goes wrong is increased as well.
That is what the insurance companies want when they look at coverage .... "Is the Captain of the aircraft experienced enough to cope..... has he the experience to NOT PUT THE AIRCRAFT IN PERIL by making an incorrect command decision ..... and sometimes it has been recognised this requires time in the aircraft doing the job.
Yes I know this is the "Catch 22" in the industry ...but believe me you can cause a mountain of grief very quickly by making that wrong decision ....just look at a couple of current threads on this forum to see what I mean.
Cheers "spinny"