In the time honoured tradition of excuse makers, it's all the fault of the previous people. Except this time, it really is. Honest!
Old UA was a distributed system with one process per logged in user. The first copyright in the code was to Brandon S Allbery at Bradford University in 1984 (and it may be older than that), although the original was significantly different to the interface we know today.
Manchester University Computer Science department began using it around 1991. The joys of NFS meant that instead of all copies running on a single machine that people telnet'd into it was run all over the deparment, which didn't help speedwise. Several restrictions were placed on normal users to limit the damage (10 minutes between logins, 4 hours max) and on the system as a whole (20 users maximum), which also had the useful advantage of making people actually aspire to being editors :-)
Factors both processing and polical meant moving to a dedicated machine at Manchester University Computer Society in 1995. The speed improved, but times were changing. The code was bloated and had been worked on by a lot of people who didn't document or comment stuff. Something radical needed - a new angle, a new codebase, a new UA!
Only trouble is, you can't just replace a system like UA. For all its quirks and foibles, people liked it. A lot. So much so that it had to look the same, work the same, feel the same or they might as well not be using UA at all. So that was the first of several challenges. Through 15 years of development UA had grown to some 12,000 lines. You don't just write that much code in an afternoon and fix the bugs after dinner. Many people found this out to their detriment and a web page is dedicated to their efforts.
The current system in use is UNaXcess II. Work began as a series of emails in June 1998 about a good way to represent hierarchical data (you find out all about the internal workings elsewhere on the website). To cut a long, technical story short UA2 went live in September 1999 after a year of development. Now into its 4th year of operation the code has been modified extensively since the rollout and is now at v2.7
It turns out old UA wasn't Y2K compliant either...