tl;dr: "most journals are sold under “big deal” contracts in which publishers in effect give libraries all or nothing offers to subscribe to their whole catalog in a given field rather than allowing librarians or consortia to chose their subscriptions journal by journal. See the work of Ted Bergstrom and Preston McAfee[1] on this for more details. What the exact incentives of the publishers are here is not entirely clear. They seem to care weakly about the over-all quality of the bundle, but they get little or no extra revenue from increasing page counts. Thus, the decision to starve journals for pages in light of strongly increasing submission rates is driven by the commercial interests of the publisher and is entirely contrary to the mission of fostering scholarly communication. This is a new, and I think compelling, reason to try to reclaim scholarly communication back from commercial publishers and into the community of scholars, and is the main point I would like to be taken from this letter."
edit: additional summary
The letter starts with the position that journals have become irrelevant for scholarly communication, which has been my experience in Economics, although I get the impression from other people here that that's not universal across fields. But (again from TFA), despite being irrelevant for communication, journals are important for promotion, hiring, etc., and that their low acceptance rates are causing serious problems. It then argues (and this is the quotation I pulled out) that commercial publishers are a big reason for those low acceptance rates, so they've got to go.
[1] http://www.econ.ucsb.edu/~tedb/Journals/BundleContracts.html