You can't really ship all of the grsec, because some compile time settings break userspace apps. Other options disable things you want on laptops, like power management registers. Basically your need to opt-in to grsec and you need to know what to configure afterwards. Or enable only selected few features and not get the whole protection.
On top of that, you can't use the latest kernel version until grsec is ported to it. And probably more reasons I don't know about yet. Basically, it's not one-size-fits-all solution - Ubuntu would create more problems than solutions if they just dumped it on users by default (or at least without a proper migration path with lots of deprecation time).