reason is that there are not many people who can do this and if there are any they are happily employed
but this should not discourage you from looking for one, i am sure they will me someone who whats to do this. or you hire a good
JS/html/css guy wth no proir exp. in Zimbra Dev and iam sure with little searching on fourms and looking at existing zimlets code he/she can be up to speed...fast.
Raj