In this instance what is holding them back isn't necessarily that you are European. They are probably more concerned with some or all of the following (plus what I forget).
- Education? Where did you learn how to program/run a business (typically this can be counter-acted by evidence of success already)
- Location? Yes, you are visiting California and are "ready to move". But you haven't moved yet. You're still located in Europe - that's the bottom line
- Incorporation? You are not incorporated yet. You may say you are willing to, but they will typically not care much unless they know instantly you have the next Google/Facebook.
- Legal resident status? As you have neither moved to California nor incorporated there is no true indication if you will be allowed to get a visa to work in the U.S
I would suggest taking the plunge if you really need the capital. Get the visa, come to the U.S. set up your HQ out of your shared apartment and incorporate. Then look for VC.
Or just go to European VC firms - there are plenty of them.