Don’t worry, nobody is at fault . And NASA is not using a US rocket to lift it to orbit.
The James Webb Space Telescope (JWST) is an ambitious thing: more sensitive than the Hubble Space Telescope, but intended to be placed at a location (Earth-Sun L2) where it will not be possible to service it. Everything has to go exactly right and it’s difficult to test all components before launch. There have already been some physical problems, including screws coming off and small tears in the sun shield.
As for the use of Ariane 5 for the launch, that’s essentially free for NASA, as a contribution by ESA to the project. One also has to take into consideration that there were fewer possible launch vehicles back when the telescope was designed, and some aspects of the telescope may have been designed with this launcher in mind, making it impractical to switch rockets at this point.
Still, yes, JWST has been a poster child for over-optimistic planning for a long while. This article gives a good recap of the problems. I wouldn’t be surprised if the project were cancelled before launch and then used as political fuel.
ETA: The “nobody’s fault” comment refers mostly to a work slowdown due to the COVID-19 pandemic.
Surprised it has not been cancelled by now. Must have some powerful backers in Congress.
The JWST is one of the most important scientific tools we have ever tried to build.