My guess is that companies doing multiple accelerators tend to be those playing startup or placing too much faith in external factors making them successful. Networks and alumni and advisors are important, but if you're optimizing on that front, you're distracted from the real work of building a company.
The core of a great company is the same: work your ass off building something your users love. No accelerator can give you that.
This is my best guess too. As a founder of a moderately successful startup that has never been through an accelerator or taken outside investments I have had to be 100% focused on the business and its success by necessity.
Why there is great value in a program like YC, it appears easy as a founder to fall into the trap of thinking getting into YC is an end in itself. This can’t be good for business success.
The core of a great company is the same: work your ass off building something your users love. No accelerator can give you that.