One of the great advantages of the mini-excavator vs. a tractor mounted backhoe is the operator can stay in one seat to re-position the unit. Both have relatively short reach when digging - 5 to 10 feet, usually, from the machine itself. So you are constantly moving/repositioning the unit to dig further. But the mini-ex lets you immediately go from digging to moving the unit, while the tractor requires that you do something to change where/how you sit each time you have to stop digging and move the tractor a few feet.
With many tractor backhoes, you can step from one seat to the other without dismounting the tractor. It's a bit harder than flipping or swivelling a single seat, but it can be done (unless it's a cab tractor, I suppose). If the backhoe is on an HST machine, it's possible to use a broomstick or something to operate the go pedal from the BH seat, allowing you to creep a few feet, such as when digging a trench. But, if any tractor or backhoe manufacturer ever came up with a better and safer approach to moving a backhoe-epuipped tractor short distances without flipping or changing seats or using broomsticks, etc., they would probably attract pleny of buyers.