Panoramic photos tend to be very large files. Even if there weren't a file size limit on uploads it would take a while for those of us with slow connections to view the threads they were posted in. I use Photoshop to optimize my panoramic photos before uploading them. This reduces upload and download times and doesn't effect the viewable quality of the image (although it would affect the quality if you were to print these optimized files).
I use this utility sometimes as it's just faster than Photoshop if all I'm doing is resizing and not touching things up. Even if you select default sizes it won't distort panoramic photos, it simply changes the width to what you've selected and then scales the height appropriately. Generally resizing an image to a width of 1920 pixels is good enough. It'll bring the file size down and most people aren't running native resolutions on their computer monitor higher than that.
My guess, as Gallowbraid already mentioned, is size. The 'Upload File' button has a size restriction of 2048 pixels in either width or height. The pictures I post are 1200 pixels on the long edge. More than happy to walk through some stuff. Just let me know.