Enlarging an image causes loss of detail, resulting in a blurry or pixelated print (ugly!).
The more pixels your image has, the more detailed it is in terms of data. You can't take a limited amount of information (a small number of pixels) and turn it into more information. Pixels have no physical size, but they do have a limitation: they are the smallest element of individual information about your digital image.
If you try to “stretch” the information available in a small file to fit a large printed dimension it will result in a blurry, pixelated print like the one below:
We don’t want this to happen to your images, so we’ve set our lowest resolution limit at 150 ppi. This also means that we cannot enlarge an image past its dimensions when set at a resolution of 150 ppi – the print size shown in our preview window for 150 ppi images is the largest we can print that particular file. However, this cap can make designing your image fairly simple: just create your design at whichever size you’d like it to print, when set at 150 ppi.
See also: How big should my image be?