Can anyone explain to me why I have to take off my distinctive Infantry items for my DA Photo.

You have to take off anything that is not a permanent award and you can change MOSs so it would not be permanent. This could be the only justification although I don't know about Infantry stuff specifically in fact I though while you were in an infantry billet you should be able to wear it. I do know that a CPT looking at the MAJ board who was IN and switch to Signal was non-selected because he was wearing his old infantry cord. Either way it is worth following the rules vs. getting non-selected.

It makes no sense to me. The rationale that "all must look the same" doesn't fly since you compete for promotion with other 11-series soldiers, not with cannon cockers or rotor heads. If you are assigned to an infantry slot, and hold the 11-series MOS, the distinctive accessories should reflect that.

If your situation changes (reclass or transfer to a non-infantry position), updating your photo isn't a major challenge.

There are no military orders published for the award of the Infantry cord or discs. You cannot wear anything on your uniform for a DA photo that you do not have permanent orders for except

SSI/FWS
Unit crests
Unit awards (received while assigned to that unit)