Back in the day, we didn't have a ton of choices.
Biggest factor is here - ARMS made one of the first successful attempts at an integrated BUIS with a solid mounting rail that was capable of taking a ring mount, and capable of taking repeated abuse.
For regular troops, these were what you could get without breaking the bank.
At the time, drop-in quad rail replacements for the stock handguards were not common, and the Knight rail system was not fully fielded with the M-4.
The ARMS BUIS is generally co-witnessed in the center of the Aimpoint window, using the Aimpoint riser with the SWAN mount.
This is old school gear for an M-4, when the rifle was first becoming common in the field.
