Because there are a lot of people who dislike Spektrum, for a perceived lack of reliability. Perceived is key. I've never had a problem in my 6 years using their stuff. 3 radios, dsm, dsm2 and dsmr receivers, noisy environments, on-road, off-road, 5 turn to 85 turn motors, Futaba, Savox, Hitec and Blue Bird servos, in rigs weighing anywhere from 200g to 4kg. The only time there was an issue was when one of those hoopty P.O.S. Flysky radios was being used nearby. But in that case, everyone had problems.
I can think of one weak point a Spektrum receiver, especially an older grey one, as Grandpa mentioned. The solder joint holding the antenna to the circuit board gets brittle. I don't suspect that's the problem though.
So, getting away from the radio, if all else is equal and others are running the same hardware on their cars and you have tried multiple cars of your own, the common factor is your batteries. How are they holding up? Are they experiencing a severe voltage sag when run? Have you tried running with the low voltage cutoff disabled?
Your motor/ESC temperatures don't seem that bad. At least not bad enough to set off the safety.
Jim