What you do with grub-install only tells grub to reinstall itself in the partition. It doesn’t tell Ubuntu package manager to install it there by default.
To do that, you need to reconfigure the package. Something like: sudo dpkg-reconfigure grub-pc. It should ask you where to install the bootloader and remember the setting. Example screenshots here: http://askubuntu.com/questions/135741/how-should-one-set-the-grub-pc-packages-install-devices-debconf-setting