Some of the reasons: many people (including experts) argued that the project could have been done for half the price with almost the same effect by upgrading the existing station above ground instead of building an entirely new underground station, for example. Costs kept increasing – nothing new for big public infra projects, of course. But when a multi-billion euro project slowly triples its budget, people start asking questions.

That way it also took away funding from other smaller necessary projects. One should consider here that DB (railway operator) has been shutting down smaller, rural lines for decades making it harder and harder to rely on them, when you don't live on the main intercity network.

There were ecological concerns about the planned changes to Stuttgart's inner city layout and how it affects the already bad micro climate.

Plus there was a general sense of the project being pushed through by stubborn DB officials and state government as a kind of vanity project despite the aforementioned concerns. They acted completely tone-deaf to the protests and in one instance used excessive police force to crush a peaceful assembly. Just altogether bad topics, which did not make the project more popular.

In short, it's very expensive and it will quickly reach peak capacity and need expansion.

In similar vein there are many smaller projects which are just waste of money for no benefit. There is even a whole tv-series of such obscure failed projects, as also regularly reports of the fails of Germanys Government in handling Money. So Germans have very low trust in public projects, especially when they are expensive.

I guess such views do exist in most countries to some degree, but this is something that specially triggers German mentally, which usually more composed and on the peaceful side. Similar behavior surfaced in the last year with the different pandemia-rulings, and now with the vaccinations.