they might think of website compatibility ... gif is viewable on all browser (that has gui ... hehe) ... flash is not, it need for plug-in.
some user can live without fash plugin so they dont install it ...
anyway ... for more compatibility in mind ... i guess they better use still image ... still image also can do wonders if u design it right
anyway ... u might want to suggest that to them ... :P maybe got something back for your kindness ? .. hehe