Here are some:
1) easy to implement/make the change.
Still be trigerred by winning a combat. SO should be easy to make the change in the programming aspect.
2) Like I said, the next 20 turns of Golden Age is insured by the Great Leader, because the Great Leader is has very big influence over the game. The funny situation of American's F-15 vs Zulu landing won't happen. Even American is a weaker force then, with the faster building ability and the GL's unlimited grouping ability, it will get big enough army soon.
3) You have better control over when do you begin the GA.
4) I guess Firaxis implemented GA as the way it is because they want the Golden Age happened roughly corresponding to the history of that CIV. GL triggering will help them to keep it. Therefore it's more realistic that they might use it... But hey, I never be lucky enough to win a lotto or so. So I just speak out my thought.
5) Actually, when I got the idea when I proposed the NOBLE leader implement under Monarchy. GL will help you with a early Republic, which could be true Golden Age.
Are they good enough reasons?