> Let me tell you that when Tesla entered the market as a serious player, the rest of the industry did not intend to let Tesla have their say nor to collaborate with them. Tesla had to make their own (better) design and find their way through. They could not wait for standardization when the rest of the industry was fighting tooth and nails against the transition to EV.
Supercharger came before CCS. It's always easy to create a proprietary standard; mostly because you don't have to debug incompatibilities between different interpretations of a standard. (Or deal with different company's opinions about what's important.)
Tesla has always been a member of the CCS consortium. As far as why they haven't adopted CCS in the US, I haven't seen an official answer; but we can look at the slow rollout of Tesla + CCS in the EU: It takes longer to debug equipment built to open standards than equipment built to a proprietary standard.
One thing to note is that CCS supports faster charging rates than Supercharger. It supports higher voltages and higher current. This is much more important than a thinner handle.