I think this is the wrong forum for you,
But I would say it has to do with heat. The larger the motor the more more windings, the more heat. The motors cool themselves with air fins. If they are starting and stopping often, with short run times they would build up a tremendous amount of heat, heat kills.

Paging electrical engineer Mas.

just-a-pb is spot on.

Most any motor requires quite a bit more current to start than to run. That is why you will usually see two amp ratings for larger HP motors; "Locked Rotor Amps" (LRA) and "Rated Load Amps" (RLA). LRA can be as high as 10x RLA which is why starting creates so much heat, especially for high HP motors and high starting torque applications.

Residential pool pumps don't require as much torque as say a compressor which is why you normally don't see LRA ratings on a pool pump. Also, the HP is much less so not as much of an issue.

One of the better explainations of this can be found in the link below.

Here is a link that might be useful: Motor sizing

I would also add....in addition to the high current and high heat during the first 100-120 ms of a transient start event, the suddent surge of current through the stator windings also causes movement between the wire turns. This movement is limited by varnish between the wires.

Each start event causes high current, which causes high heat which heats the varnish. Multiple start attempts with hot varnish starts to break down the varnish, eventually causing turn to turn wire shorts, arcing and motor failure. Limiting start attempts on any motor is good regardless of motor technology, 3PH, 1PH, permanent magnet, etc...they all have induction stators. The differences in specified start attempts per hour relate to the thermal mass of the stator and the heavier the stator, the longer it takes to cool as pb indicated.

Hope this helps!