If I could contribute my penny's worth, people tend not to read very long papers and my experience is that it can often by quite difficult to find somewhere which will publish a very long paper, even if the result is significant.

For example, the two proofs of the Riemannian Penrose inequality (an important result in general relativity) were uploaded to Arxiv at least a good two years before being published in an academic journal, although I don't know if the length of time might have been due to the time needed to verify both proofs, which are very long and technical (the second proof is almost 90 pages long).