Probably because it is unsupported. Make sure you read the "resource specification" section in the help file which explains the required format. One important part of it is that it has to be 8-bit color (256-color indexed palette mode). Also it's best to use the import function to select the transparent color.
I was listening to songs by Chuck E Cheese's voice actor (Jaret Reddick from Bowling for Soup) when the meds I was taking for my stomach virus apparently stopped working. I can't take my next dose until morning, but it's no use anyway. I'm officially sick again. Of course, I'll definitely take the next dose either way, but it's useless now.