No strong reason - there's a thread
here with a couple of images of the same pages in LRF vs EPUB so you can compare if you need, but the main differences are the default font, the justified vs ragged right side of the page, the page numbers & the italics.
Epub is a zipped container, so theoretically you can tweak it yourself, but in practice, I suspect you'll need to know the details of the format to do anything worthwhile, as that's typically only needed when an automates conversion has failed!