Why is this image being rejected?

The problem with raster drawings is they can't be rescaled.   If I wanted to print this on an XXX large T-shirt, the image would be too small.  If I tried to make the image bigger to fit the T-shirt, the image would become pixelated and very poor quality.  For this reason, drawings should be submitted as vectors to reach a wider base of customers.

Use Photoshop rasters for photographs. 

Use Illustrator vectors for flat color drawings, logos, text, icons and infographics.

Hope that helps.