Lilo is simple and "just works," which is probably why it is the default Slackware bootloader. Grub is very powerful and flexible. You don't have to rewrite the MBR every time you change things (to which the hard-core Slackers say "so what?").
To me the most compelling thing about grub2 is loopback support, letting you access kernels and initrd's on image files, boot live CD's from iso's, etc. It can mean a lot less CD burning.