You can - it's not where you'd think - "click the difficulty tab" and it resorts. :(
It's on a small yellow button over on the upper right that says "sort by" - there is a drop down tab from there that will let you sort by difficulty.
I'm hopeing they beef up the search and sort myself - I want to search by tuning
"Half Step down"
Things like that so I can work songs without retuning so often... ;)
It's an area that the site could stand some work on... :)