Vampire Weekend is a more recent band than MGMT, having been formed in 2006, while MGMT was formed in 2002. This means that Vampire Weekend has had less time to build up a fan base. Additionally, Vampire Weekend's music is generally considered to be more accessible than MGMT's, which may appeal to a wider range of listeners.
On the other hand, MGMT has been praised for its originality and experimentalism, which may appeal to a more niche audience. Additionally, MGMT has been around longer and has had more time to build up a loyal fan base.
Ultimately, it is up to each individual to decide which band they prefer. However, the general observations made above may provide some insight into why one band may have more fans than the other.