Well, "we" is very precise -- it's an EU program, and this reactor is not.

As far as "why", you can take your pick of:

- ITER's design is older, and maybe could be considered "lower risk" (that it will work at all)

- Sunk cost

- Academic jobs program

- Money has already been allocated to member states, and none are happy to give that up

- Big changes take time

I'm not optimistic on stuff like this, so IMO it's another JWST, but it's not totally crazy to keep working on a plan (for a while) when new avenues of research arise.