Thermal limits are mostly voice coil size. They use a certain diameter and length of coil and well as layers and that makes most of the difference. Former material, gap thickness, and pole vents and forced cooling tricks are other things that companies use that can help some. My old AV15h was rated at 1k and was a 2.5 coil, that was due to the very small gap so the the coil was always very close to the motor which has better heat conductivity than air. Other speakers use wide gaps, but force the hot air out as the speaker moves. My new speaker is rated at 2k, it uses a 6inch coil and very little in the way of special tricks, other than a HUGE pole vent (soda can size, srs) and the coil is wrapped around both sides of the former, inside and and outside wound. Also some speakers make the formers black to absorb heat better, or make them from metal that absorbs heat, mine are fiberglass, which isn't very conductive, had they went with a black coated metal former, it'd probably be rated at 2.5k.
Companies rating arent' consistent either. Anyone who puts over 3k on a powerhandling spec is at the very least being misleading. Clamp a real 3k to any speaker and it wont' hold up that long, at least not if we are talking hours, all day every day abuse. Pro audio uses a very strict standard for power handling and their 4inch coil drivers are only rated around 2k, and those are built to handle power very well for long periods, longer than we'd likely see in a car. Just as all top of the line subs claim 30+mm of xmax, I'd bet if you put them on a klippel most don't break 25mm.