1) All luminaires are UL rated based on their originally tested lamps. As soon as you put an LED retrofit lamp into an HID luminaire, even if the LAMP is UL rated, the luminaire is no longer listed using that source unless it was specifically tested with that exact lamp and that exact luminaire. If anything goes wrong, whomever authorized the use of the "incorrect" lamp would be at fault.
2) With HID, the optical performance is very low with very little aimability and approximately a 30% loss of light within the luminaire. This is overcome with HID because it produces so many raw lumens, that even though many lumens are wasted, it still provides adequate light in most cases. LED replacement lamps suffer from that same 30% optical efficiency, so you would have to derate the lumen output from a replacement lamp by 30% to find the equivalent to an LED luminaire designed from the ground up. That is a pretty high penalty to pay.
3) The optics of the HID allow for the control of about 60% of the light output. That means that there is a high concentration of light directly underneath the pole, but very poor spread of light. With an LED luminaire 100% of the light can be directed, so the light can be placed everywhere we want it, and nowhere that we don't. Uniformity matters.