....I'm wondering why a second vaccine is required ...
Short answer-the response to each vaccine in each species is different. It's really one of the beauties of the science how much variability you can find and then search for reasons.
More realistic answer-people don't like to get shots so there is a high economic incentive for a manufacturer to develop a vaccine that has reasonable efficacy in one shot. With flu vaccines in many people, you're also building upon the immunity from the vaccine in the previous year.