​Why does IDPA prohibit open carry in its matches?

​Why do the rules for IDPA (International Defensive Pistol Association) prohibit open carry in its matches? 

1.2.1 Equipment Principles

A. Concealable: All equipment (except flashlights) will be placed so that it is not visible while wearing a
concealment garment, with your arms extended to your sides, parallel to the ground.
B. Practical: All equipment must be practical for all day concealed carry self-defense, and worn in a manner that is
appropriate for all day continuous wear.

3.1 Concealment Garments
A concealment garment is required for all stages unless otherwise specified in the stage description. This includes standards and limited stages.

Police or military personnel may use actual duty gear in Tier 1 matches. See definition of duty gear in the Equipment Section. Use of duty gear eliminates concealment requirements.

Additionally, are numerous prohibitions of things like compensators, add-on weights, extended magazine releases, holsters not practical for everyday concealed carry, etc. Essentially, a gun used should be in as close to factory condition possible.
 
The short answer is to keep IDPA shooting as close to a realistic self-defense scenarios as possible. As Ken Hackathorn, one of the originators of both USPSA and IDPA pointed out to Ian MacCollum of InRangeTV, USPSA grew away from its tactical origins to become a gear and reloading manipulation competition. Most of the gear, holsters, and firearm modifications for USPSA has more to do with speed than defensive or tactical considerations. IDPA was a reaction to the “equipment races.”
 
Specifically to open carry, IDPA requires concealed carry for several reasons. At the time it was started, open carry was not a usual method of everyday defensive carry when even concealed carry was illegal or highly restricted in many states.
 
More practically, there is no real way to distinguish regular open carriers from those who would only do it for an advantage in competition. It is guaranteed that if an everyday concealed carrier could speed up his IDPA match times by openly carrying on the range, they would do it, but would lose out on the skills like drawing from concealment. IDPA after all was intended partially to help self-defense shooting skills in realistic scenarios.

8.7 Duty Gear Exemption

Police and military personnel using the duty gear exemption are exempt from:
a. Using concealment garment
b. Holster design and placement requirements (other than listed above)
c. Ammunition carrier design and placement requirements
d. Belt design and placement requirements

Law enforcement and military are exempt because they regularly carry openly. This allows them to build and practice skills they can realistically use rather than just cutting down on their times. If civilian open carriers came with a mark or something to distinguish them as bonafide open carriers then maybe IDPA would let them do it too.
 
So IDPA is not putting down open carry. Rather, they are keeping the matches as close to “real world” scenarios as possible by keeping the “gear queers” from ruining the sport.