Because a neutrino beam can't be bent.
Photons can be made to follow an optical fibre if the angle they make with the sides of the fibre is low enough for them to all be reflected back into the cable.
However, neutrinos are particles that barely interact with other particles enough to be detected, which is why they can travel 732 km through solid rock. The chances of reliably deflecting such a beam in a circle are approximately zero. Similarly, they carry no electric charge so magnets won't deflect them either.
To show how unreactive neutrinos are, Its been calculated that a neutrino beam can penetrate a lightyear of solid lead without loosing more than a few percent of its brightness and they can whistle through the sun almost without noticing that it was there.
So, as collisions don't deflect them, they can't be reflected by anything, and magnets or electric fields don't affect them, you have no choice except to design the experiment around measuring a straight beam on neutrinos.