Because its a phone. It can't process all those images into an animated file all the time.
If you are using a built in browser instead of firefox or chrome, just be happy you can load web pages.
My iPhone 4 can handle 10-15 Gifs on one web page just fine.
Chrome displays animated GIFs perfectly for me (on Android 4.3).
Have you already tried a different mobile browser, like Opera Mini?