Because with Tieflings the prejudice against them is wrong. With orcs, it’s justified within the setting. The problem isn’t that prejudice exists in the fictional setting, it’s that, in some cases, the setting treats that prejudice as justified.It speaks to many of the same concepts we covered in the massive "UA Thread of 3000 Posts" @Oofta
In the same way as people are not happy about Orcs, or Gnolls. Its establishing a singular view on a lineage, which is negative.
MY view on Tieflings remains the 2e/3e/Planescape vision. I reject the current one outright, and have since 4e dropped, but if one views the current Tiefling as acceptable, I see no reason why Orc's would not also be acceptable.