It wasn't a duopoly at the time though the first public release of webOS followed Android's by just a few months, Symbian and LiMo/Tizen were still players in potentia, …. And between the innovative yet intuitive interface and web-based tech[0], many people were super excited by webOS.
The issue is the Spring exclusivity (distant third) hurt, Palm flubbed the original hardware quality-wise, and they didn't have the breadth (or balls?) to build a consumer-oriented version (purely software interface) and leave the hardware keyboard behind.
[0] though that also caused real issues wrt performances.