Honestly, it's mostly because they pay more. They're more selective and thus have better engineering talent meaning Twitter engineers on the average are going to have a better engineering skills than Tesla software engineers
Tesla engineers likely are drinking the Kool-aid or were not talented enough to get offers at better companies.
Lastly, it's not "just tweets" at their scale. That's like saying Netflix is just storing some video files in CDNs.
I get twitter has some issues of scale to solve, but its tweets, even netflix would be far more challenging imo.