It's fact pure and simple.
1) Because it's easier and faster to deploy your knife
2) Because you are less prone to cut your fingers when you are reaching into your pocket since the blade is facing away from your hand and trapped by the back part of your pocket to prevent it from opening.
3) Because the strong majority of knife nuts prefer it that way. If you do not prefer it that way, you either have no common sense or simply have a bad and inefficient habit of carrying your knife the wrong way and need to correct yourself.
Point 1 is opinion/personal preference. Speed is very, very close, but I was playing around with my PM2 in both positions, and I can definitely open it faster tip-down (spydie drop vs. middle finger flick). In point 3, you mention that one might have a bad habit - but one could also make the argument that people with tip-up are only able to open theirs faster because they're used to it. This is also a discussion of the PM2 in specific - certain knives I can definitely open faster in tip-up, but here tip-down is faster. You need to try both ways on each knife (if possible) before you can make an informed judgement. Plus, different sized hands, etc. You can't just assume your exact circumstances and preference must be universally right for people with different hand sizes.
Point 2 has merit, but is situational. Tip up carry, pushed to the back of your pocket, will trap the blade there, but tip-down turns the point of the blade away from your hand, so if it did manage to open (maybe you don't like it pushed back in your pocket), you can't stab yourself with the point.
Point 3. If a majority opinion is indicative of fact, we're all wrong for carrying a knife in the first place. Majority preference only shows what the majority prefer - while it can be indicative, you need to break down WHY people like it for actual reasons.
Long story short, I still think it's a hilariously small difference, but I don't think you can throw that out as a fact. Neither carry method can be objectively said to be universally better, only what works best for us personally - both have pros and cons, and it depends which are important to you. For example, I actually carry my waved knives "backwards" with the clip in the left-hand position on my right side, because I like that I'll never need to check behind me in a crowd before drawing a knife, even though it's ever so slightly slower that way.