Even journals have limits, because they don't want one article to take up an entire issue or 2 or 3 issues, if the article is long enough.
But, with conferences, time is another important factor because there are deadlines to be met (which is not an issue with journals, because if your article does not appear in an issue, it can appear in the next). It is hard to find reviewers even in journals, where the editors are quite lenient in time allotted to the reviewers, what to speak of conferences, where the reviewers are in time crunch as well. Can you imagine a reviewer getting a 100 page manuscript to be reviewed in a month? If I got such a manuscript, I would decline it even if infinite time was given.
Conferences also usually publish proceedings, so again the length of article becomes a consideration from cost and binding perspective (you can only have so many pages, before the binding is unable to hold the pages together). But, even if publication were not an issue, finding reviewers is quite hard and it is 100 times easier to find a reviewer for a 6 page article than a 100 page article.