8,623 votes and 263 comments so far on Reddit

I think it’s because (at least when you leave RAW) you can customize your character the most with tieflings. When you break it down in terms of appearance (especially for stylistic graphic design) Elves are human-like in appearance but with pointy ears, dwarves/halflings are short humans (with/without beards), etc, where as tieflings you can make any color, add/remove horns, hooves, tails, etc.