But everyone you speak to still says it represented marvellous science. So why did it lose out?This is an excruciatingly difficult experiment to do, so it will be an astounding effort if they can detect such gravitational wave. But like the Higgs, it is an aspect that needs to be verified and confirmed for GR (as with the Standard Model for the Higgs).
There're probably a few reasons, and I'll try to summarise the comments that have been made to me.
One was the price tag. Even in its remodelled format, the mission would have cost Esa more than 1,000 million euros (the Lisa/NGO team disputes the reality of this figure) and this was substantially above the ceiling of 650 million the agency had set.
Another reason was launch readiness. Esa's executive did not believe the mission could be made ready before 2025, and it wants to maintain a certain flight cadence for its science projects, ie Esa needs to be seen to be doing stuff regularly.