Well language change is very complicated, but I wouldn't say that we 'lost' a European accent, just that European English and American English evolved differently. The standard American English dialect is also hard to chart since American English has a huge variety of dialects and accents such as city accents: 'Bostonian', 'philadelphian', etc, regional dialects: Appalachian, Midwestern, southern, etc, and sociolects like African American vernacular English and 'valleyspeak' or upspeak like you mention. The development of valleyspeak is more recent and well documented. I think the name comes from a Frank zappa song where his daughter speaks in the background, but in general the kind of speech is associated with the California valley religion, but parts of it have made it into general American English, such as the use of like as a hedge. As to why American English and valley speak arose to sound like they do is more random and complicated, I'm not sure why the valley speak developed it's characteristics features of the use of like and the tendency to raise the tone at the end of a sentence.