SpaceX is doing the classic statistic thing[0], making spacecraft stronger where they explode until they don't. It's more like a hyperparameter search and less like QA for individual parts.
SpaceX is just doing what the Soviets did. The Soviets preferred to launch and learn just like SpaceX does today. Soviets also ran on a much smaller budget than NASA and preferred simplicity over complexity and constant tweaking.
The Soviets proved that methodology works and SpaceX continues it to this day. I have a feeling most that follow SpaceX think they are trying a new revolutionary approach here.
The N1 didn't really fit the testing driven development approach, they couldn't static fire the flight engines, they ended up relying on test firing extra engines from the same batch and assuming all in a batch were the same.
I'm no rocket expert, but the Wikipedia article you linked seems to have an idea about that:
>Adverse characteristics of the large cluster of thirty engines and its complex fuel and oxidizer feeder systems were not revealed earlier in development because static test firings had not been conducted.[9]
Time and money. Powers at be decided to stop funding the project. If SpaceX goes bankrupt next year, they could fail too and it won't have anything to do with the technology.
Strategically, they were caught with their pants down when the US demonstrated they were serious about going to the Moon. The soviet space program was great at sending cosmonauts to LEO, but they had no ability, and no desire to spend the time and effort to develop the ability, to go to the Moon.
They planned and expensed the N1 as if it was another LEO Soyuz variant.
The N1 was always doomed to fail because the USSR spies had been found and were given misleading information and plans for the US space program. The US intelligence community snuck bugs into the N1 design by sneaking them into the designs their spies thought they were stealing.
[0] https://upload.wikimedia.org/wikipedia/commons/b/b2/Survivor... :)