Some people are nervous about installing anything that's third-party, especially after the repeated warnings about "Third Party Tools" in the ToS and such. And you know how Windower was back during FFXI days, where it was a "don't ask, don't tell" kind of affair, where I don't think anybody got banned, but rumors flew around of a few who did, and there'd be a "yea huh/nah-uh" thing going around whether or not that was actually true.
A lot of people stepped back and said "I dun wanna risk it....." and just didn't use it at all.
That and at the end of the day, I think the only people who really care about parsing, are the people who want to get into higher end endgame content. Your average joes, they don't really care as long as stuff dies and quests get completed.
It is sad that Yoshi-P is hesitant to include DPS meters as part of the game, but then if you look at WoW, you'll understand why. I have personally seen toxic players berating each other and e-peen waving, it's ugly and I'd rather not see it here in FFXIV. The fact that only some endgame raiders use parsing tools, and the fact that said players are rarely found (or they are rarely bothered to care except for the most egregious examples of poor play) in Duty Finder content means that there's a nil chance that you're ever going to get crap thrown at you if you put out a less-than-stellar performance.
I'd rather have this... than have what goes on in WoW. There's a reason I don't do dungeons in WoW all that often. That would be one.