How would they do that without divulging the test questions? To offer more than twice a year, they'd have to write quite a few more test questions, which they don't have staff for. The exam writers and graders do it as a temp gig.
They also don't have permanent test location facilities. I took the exam in the basement of a convention center. Keep in mind that most states write their own bar, and especially with smaller states, the number of takers may only be in the hundreds. Also, it may take an entire day to take an essay exam consisting of only 5-6 questions- which makes options like an adaptive exam not possible. Centralized test centers and/or an adaptive exam might work for the multistate bar exam (which is a component of every state's exam), but that's 1/2 or less of the exam.
In most states, the bars actually aren't that well funded. Despite popular belief, a lot of lawyers are far from rich, and bar dues need to be set so the public defender making $40k a year can still pay them.
Things are not as simple as they seems at first. - Edward Thorp