history - Why does legal English continue to remain archaic?

Perhaps this is a question for Law.SE if one exists, but I am asking here as there are other nice questions on English history.

There is some historical development account presented in Wikipedia, but I find it grossly inadequate to answer the question. English has always been a language that borrows and enriches itself, but retains its original flavour all the same. Why has a similar linguistic edification not happened with legal English? The question, in its elementary form:

• Why has legal English simply not moved on?

Answer

Because every attempt to change it makes the law more complex and more expensive. The archaic terms, like 'plaintiff' and 'writ' had clear definitions, partly because they had been hammered out over generations. A well-intentioned attempt to make it easier for the layman to understand (which every new Government tries to bring in) replacing them with 'claimant' and 'claim form' ends up causing confusion at the margins: if one partner in a marriage starts the divorce process but makes no financial claim while the other does, which one is the claimant? A writ was different in important respects from an originating summons; if both are replaced by a Claim Form, does the distinction survive? And, of course, confusion at the margins is where lawyers are needed, and where it gets expensive.

This is not pure guesswork on my part; in 2000, the British courts made a huge effort to simplify the civil courts' language and procedure. Everybody agreed that it was overdue, as the Rules of the Supreme Court were over a thousand pages long, and had to be re-issued every two years. Several of the brightest people in the country spent years rewriting the rules, and I was one of the people who had to try to make it work. It failed miserably; it turned out that words mean different things in different contexts (as any contributor here knows), so to replace one archaic term needed several near-synonyms, and each occurrence had to be scrutinised by the whole committee to see which applied. In practice, of course, this was impossible.

(As an analogy, try to debug a thousand-page program where each occurrence of each variable has to be replaced with one of six alternatives, but nobody knows which; and, of course, regular updates change both the variables and the structure while you're working. Large parts of it are safety-critical, since it bears on the liberty of the subject; and it has to be compatible with other systems across the world, in a variety of languages and with different philosophies.)

The current Civil Procedure Rule book is heading towards two thousand pages, and is replaced every year, with quarterly updates. And, of course, the expense and confusion were not blamed on the politicians who ordered the change (and moved on before it happened), but on the lawyers and civil servants.

TL;DR: Because archaic English is clear and logical, while modern replacements are more confusing than helpful.