Laws were originally written in Latin or French, and many of the common terms are still being used. Drafters were once paid by the word, rather than by the job. Drafters rely on precedents, using ancient clauses rather than 'risk' using original language. Many old documents which are now part of our law were written by humble court clerks rather than skilled lawyers. This may be because of a tradition of making each part of a bill only one sentence long.
Experience shows that shorter sentences, each dealing with only one main point, are more effective.
This does not have to mean a 'Janet and John' writing style, rather a conscious effort to make each sentence serve one precise purpose. As well as obscuring the message, this can be legally dangerous. Courts will usually assume that every word in an Act is there for a reason, and unnecessary words may be interpreted in a way that the writer had not intended. If double negatives are used, the reader has to perform mental gymnastics to decipher the meaning of a sentence. This often including using unfamiliar words where common terms would do just as well.