I wouldn't say Edge was BETTER ...but if there were ever a reason to be found (in MY opinion), it'd be because Christian was just kind of always in the Edge's shadow. It was Edge & Christian...not Christian & Edge.
Edge debuted first and then that whole Edge Vs. Gangrel thing happened with Gangrel introducing Edge's little bro Christian. At that point, it just becomes hard to try to envision Christian as being superior to Edge. Not to mention, Edge had some of the biggest spots in the matches that made them famous as a tag team.
It's kind of how Kane will never quite be looked at as the Undertaker's equal even though Kane puts in consistent work & holds his own. Chavo'll always be compared to Uncle Eddie...
...and then the height & look is a factor, of course. Opportunity knocked and Edge was there. Ultimate opportunist.
All this right here.
Also, I've just always found Edge to be a more charismatic character (don't even ask me how). Christian had his points where he was interesting ('05 with the Cena feud), but he always seemed like more of a perennial midcarder. Kinda like how Matt Hardy was better in-ring and more reliable all-around, but Jeff always was more popular. Christian took a minute to hit his stride when they split the team up, and he was always #2 when they were teaming... so it was destined that Edge was gonna get the bigger push.