Unicorn HAT, or more specifically the
rpi_ws281x library responsible to drive the neopixels requires low-level access to the hardware, hence
sudo being required.
Could it be worked around? I guess, possibly, deferring access to something else that is allowed to do that by default, not sure. Not a simple task I would suspect, certainly requiring a rewrite of the interfacing library.
... Piglow is a different beast, it drives a SN3218 chip over I2C, which requires no particular privileges, hence the reason why you can dispense prefacing the command with