I would think because the original founders spent a lot of time planning, researching, and designing combined with decent timing of Musk jumping in with money. Why else would Musk have bought them in the first place if they didn't have incredibly impressive ideas and engineering to sell? When the roadster originally came out, it was expensive, but also had a near 300 mile range which nobody else even came close to offering and boasted very impressive engineering and crash safety. And im sure a lot of that work was put into atleast the next 2 models released.

Of course the quality has fallen faster than the price over time, but initial impressions still hold on for a long time in general.

I think SpaceX's success is mostly down to throwing money at the problem. The US had tons of graduated aerospace engineers with limited places to go, and places they could go directly in aerospace fields were already committing their funding to established programs. SpaceX startup would of been a dream job for the top aerospace engineers because it was all fresh ground but with a far larger budget than 99.9% of startup aerospace companies. They weren't offered to build one piece of a rocket that may or may not get sold to NASA or someone 15 years down the line, they were offered to work on and put their mark on a completely new rocket design that was going to at the least be test launched. And im sure their early successes helped boost recruitment even further, combined with government contract to keep the money flowing.

We probably don't see many rising EV companies in the US because you need an ass-ton of capital to start an automotive company, and most people holding enough capital to do so know that try to sell cheap consumer cars that most people want is not really the highest margin business. Selling a few hundred or even a few thousand cars still leaves you with a mountain of capital requirements in front of you that your margins are going to have a really hard time climbing. And if you don't climb fast enough, good luck fighting established auto makers and their lawyers with every cent tied up into trying to scale and engineer.