The best answer besides the payment thing is probably familiarity and inbuilt audience numbers.
People are use to Kickstarter now. It's big. When you think about crowdfunding, a majority of people think of Kickstarter as the first thing that comes to mind. It's something a majority of users are so use to know. It's like the whole PC vs Mac thing. People will defend each one and say it's definitely better and all that, but a majority of the reasons is simply what people are use to.
A lot of people know about Kickstarter so there's a lot of in-built traffic due to that. That's not to say you don't have to work, but you're going to get Kickstarter backers that are randomly browsing the site that might land on your project and pledge. I don't know hard numbers and I'm going to use the internet for this, but look at both their Alexa Ranking:
Kickstarter USA : 214
Indiegogo USA : 628
A lot more people are visiting Kickstarter over Indiegogo and that adds up over time. It's not a bad platform to use, from what I can see, and they have their own success story.
For a lot of people, I think they want to go where they're use to, where they know things better and where they think they've got a higher chance of success.