hhmmmm I have heard an number of theories on this topic.
1) Confidence
By nature white guys tend to be more confident than their Asian counterparts. Majority of white guys are taught at a young age to act tough and be a man. Don't fear rejection and be aggressive. They're taught to take what they want.
Asian guys are sheltered, they are taught that they should focus on their studies and do well in school so they will get a good job and so on. So they don't go out, stay in or do a lot of tutoring. They mostly interact with their nerdy counterparts or spend their free time gaming and interacting with people online. This cause them to be a bit more socially awkward around females, especially pretty ones so they tend to put them on a pedestal
Media
Nowaday we are so bombarded by the media with white dominant males that a lot of women just find white guys very attractive. A lot of Asian women prefer them because of their white skin, pretty blue eyes and big exomorphic, build. Asians are never portrayed as tough, sexy male leads. Even majority of Asian men who are considered sexy in Asian countries try to be white as possible. They use a lot of skin whitteners and try to look pale, dyed hair and wear colored contacts.
Common Interests
A lot to do with dating is to do with common interest. It doesn't matter what your background is you have to like what the girl is into. If she's not into what you're into it doesn't matter. I do find a lot of Asian girls are more accommodating to liking what the guy likes, especially what White guys like. White girls like whatever they like, as an Asian guy you have to adapt and be more open to doing what they want.
Of course there are a lot of exception for these rules. There are Asian guys who are very confident and there are white guys who are very shy. But majority of all guys fall within these rules.
I know I sound harsh to the Asians. Being Asian myself, I've tried to date white girls before and came close a few times. But in my early years I wasn't the party type and a lot of white girls I've met loved to dance and party.
But those are my theories.
Also, I found from talking to a lot of international students, when they come to Canada to study English, that when they come here and want to date a Canadian guy they aren't thinking of people who are from Vancouver and speak fluent English. They're think of white guys. You could be Asian, born and raised in Vancouver and not be considered Canadian in their eyes.