I have seen something where the Kaminoans didn't know too much about human biology and created the helmets thinking they would do the job.
The Phase I armour proved uncomfortable for the troopers to fit in, which was due to the Kaminoans unfamiliarity with human ergonomics.
Ultimate Star Wars
However over time this proved not to be the case and the designs were modified. Starting with the Phase 2 armour where the T shaped visor started to become more rounded and reminiscent of the helmets worn by Stormtroopers.
The Stormtrooper helmet removed the down part of the T and further rounded the eyes out.
This either provided a bonus in vision, though as all helmets contained HUD this would probably be negligible.
Or they wanted the storm troopers to look different enough from the clone troopers to show that these are the soldiers of the new Galactic Empire, replacing the clones that would represent the old, decadent, bureaucratically crippled Republic.