The straightforward answer is because they like their current rules more than Robert's rules. I'd imagine literally every rule was voted on by the body in quest

The straightforward answer is because they like their current rules more than Robert's rules. I'd imagine literally every rule was voted on by the body in question, so they are all things that people supported. If you want to get really cynical, the major parties are complex coalitions of interest groups. It is really convenient for the power brokers to have arcane rules because issues can be delayed and the waters muddied around which politicians support what. Doesn't need to be perfect, it just needs to be weird enough that the average voter gets confused about whats and whys of procedural details.