Why do my gnomes not use wheelbarrows?

I have about 15 Wheelbarrows in my Kingdom, yet my gnomes barely use them.

Is there a (hidden?) minimum skill requirement or anything? How can I make my Hauler Gnomes use more Wheelbarrows?

The wheelbarrow is still a bit bugged. Sometimes your gnomes keep loading by hand while there is a wheelbarrow available all the time. Suddenly they start to use it a couple of hauls only to return to hauling by foot again after a couple of runs. Here are some sources that describe your problem and others with the current wheelbarrow system:

http://forums.gnomoria.com/index.php?topic=5111.0 http://www.reddit.com/r/gnomoria/comments/1jgyog/wheelbarrow_weirdness/

Given the date of the question you should have the latest wheelbarrow fixes. They were introduced in 0.8.48 and received multiple fixes afterwards with the most recent one in 0.8.50.

You could search for wheelbarrow in the changelog to get more details on what work has been done on them.