Why are gifs banners so popular?
 

Yes, i know, they are very hypnotic, eye catchy and have cool CTR, but GIF's are so 90', and they are back and rocking

A general rule to remember in understanding behavior is "people tend to do what works for them"

Olds School is cool.

They are small in file size, usually funny or interesting in some way and animated which draws you in.

they are catchy rather than plain jpeg files

Probably because they work across a much wider variety of devices than flash banners do.

Flash banners can have fallback embeds that works in all media sizes where html5 is also compatible. :pimp

Small file size, eye catchy , not blocked by Ad blockers, work on every device including iphones .

For example the flash banners from crakrevenue don't work 100% on Iphone...

Flash doesn't work on iOS.

As mentioned above, though, you can fallback to a gif / jpeg.