A good primer on Sparkling wines from California verses Champagne is:
https://en.wikipedia.org/wiki/Sparkling_wine#cite_note-MacNeil_pg_652-655-29
The key is in the ageing, sugar and varietals used. Also the terroir of the area in the US vs the chalkiness of Champagne.
“While many top American sparkling wine producers utilize the traditional methods of production, there are distinct differences in their wine making techniques that have a considerable effect on the taste of the wines. In Champagne, the cuvée blend will rarely have less than 30 wines and sometimes as many as 60 that are taken from grapes spanning 4–6 years of different vintages. In California, cuvees are typically derived from around 20 wines taken from 1 to 2 years worth of vintages. Champagne laws require that the wine spend a minimum of 15 months on the lees for non-vintage and minimum 3 years for vintage Champagne. It is not uncommon for a premium Champagne to age for 7 years or more prior to release. In the US, there are no minimum requirements, and aging length can vary from 8 months to 6 years. Another distinct difference, particularly in Californian sparkling wines, is the favorable Californian climate which allows a vintage wine to be produced nearly every year.[29]“
Best Napa Sparkling Wineries: Note: Just hold cursor over winery and click to get to website.
California Wines & Wineries 