Neutrinos are very weakly interacting and hence you need a massive amount of some matter through which they can pass so as to have a decent probability of getting an interaction.

Ice at kilometer level depths is exceptionally clear and transparent since the pressure (from layers above) removes any air bubbles and hence any Cherenkov radiation due to neutrino interaction does not get scattered. This means observations can be done clearly and precisely.

Finally, the rigidity of Ice can hold the sensors precisely and the depth guarantees that no unnecessary external noise from the surface can affect the measurements.

All the above are ideally met by the Ice at the south pole and hence the IceCube Neutrino Observatory was established there.

H2O is the key, not ice. Ice is nice because it's rigid, so you don't need a boat [1]. They're also working on making detector under the sea [2].

[1] https://www.amusingplanet.com/2015/07/the-surreal-world-of-n...

[2] https://news.cnrs.fr/videos/detecting-neutrinos-at-the-botto...

Among other things this ice is super transparent.