To be honest it smacks of arrogance and/or incompetence (or a mixture) from UKBA/Home Office - they'd done it before and been knocked back by the courts and either didn't think they should look at the rest of the Points Based System or were too imcompetent to do so. And look where it's got us.
The whole system is so labyrinthine that putting it all in the rules was clearly impossible, so they went for the rules/guidance approach, without doing it properly or considering the consequences. The desire to make it all rules based came from years ago and I got a kicking, when there, for telling a senior manager that without a legislative slot it couldn't be done properly. Lo and behold they went ahead with easier, secondary legislation and made a complete Horlicks of it.
By the way, if you fancy a bit of light reading, and want to see how much of a mess they can make of writing rules, try the new "Appendix FM" in the immigration rules on the UKBA website. If you understand it, give me a call