Very nearly half of HS2 - 64 miles - is in tunnels. This happened because the Conservative government was terrified of upsetting the Conservative voters along the route. The bat tunnel was nuts, but it's not that significant overall.
That is partly true but it's a downstream consequence of the ultimate cause. Those running and pushing the project in the early stages desperately wanted it to be a 400+kph route for "prestige" reasons.
If they hadn't been so focused on that, they could have built the new rails largely alongside the existing West Coast Main Line which would have been quicker, cheaper and much less controversial.
But because they wanted those very high speeds, the route had to be flatter and straighter than the existing WCML, which means many more tunnels, longer and more complex construction, more people objecting, inconvenienced etc and ultimately much more cost however you look at it.
Yeah it's more because they wanted the high speed trains to go super fast because bigger number is better, in reality it dramatically increases the impact of the track you build - corners have to be wider, gradients have to be avoided, and offsets (and the land you need to purchase for them) need to be bigger.
If they hadn't been so focused on that, they could have built the new rails largely alongside the existing West Coast Main Line which would have been quicker, cheaper and much less controversial.
But because they wanted those very high speeds, the route had to be flatter and straighter than the existing WCML, which means many more tunnels, longer and more complex construction, more people objecting, inconvenienced etc and ultimately much more cost however you look at it.