I was reading the London prize ring rules which states.
'That in future no spikes be used in fighting boots except those authorized by the Pugilistic Benevolent Association, which shall not exceed three-eights of an inch from the sole of the boot, and shall not be less than one-eight of an inch broad at the point; and, it shall be in the power of the referee to alter, or file in any way he pleases, spikes which shall not accord with the above dimensions, even to filing them away altogether.'
They're referring to spikes like cleats, to grip the mat and reduce slipping and allow for powerful punches. A long time ago it used to be more common to box in a boot similar to a work boot because of the ankle protection. It's easy to step on a persons foot in boxing, and a spike would hurt. By removing the spikes it reduces the chance of unintended injury.
Foot-stepping is a common (and illegal under most rules) boxing technique. Longer spikes would make it more effective, and more damaging.