We can deal with this more in detail in rules change season, but the brief answer is the current rules are more cost effective.
Everyone has 17mm pins already. Allowing the 19mm pins adds cost. The 19mm pins improve geometry on a lowered car to a degree. You can argue about how much, but there needs to be a good arguement for why it is needed before we'd open that door.
As far as the rear coil overs, they can get very expensive, and there is no question they improve performance. Even if they are cheap, you have to consider that making this change impacts the cars that are already built. Once a better performing option is made legal, than the 100's of cars already built have to spend money to be competitive - that's why rule changes are a big deal.