Imagine you've got a crufty COBOL mainframe somewhere, of which the IRS has lots. (The Social Security Administration has millions of lines of production COBOL, even in 2015. See: http://oig.ssa.gov/sites/default/files/audit/full/pdf/A-14-1...) Like most systems of the era, it differentiates interactive jobs and batch jobs. The batch jobs are designed to run when the interactive jobs are not, and they're not designed to be safe to run when the interactive jobs are running, because what IRS clerk needs to input data at 2 AM in the morning?

The people who've coded the systems are long-since retired. The systems are known to work but nothing is known about whether they can support requirements which didn't exist in 1970, like 24/7 access. There exists no money in the budget for blackboxing them to determine whether they are in fact safe for Causes A National Calamity If We Make Any Mistakes concurrent usage. 24/7 access is not a hard requirement, for anyone in decisionmaking authority.

Thus: office hours for web apps which are fundamentally just a front-end into an interactive job.