First, Shuckle is not terrible otherwise. It can set Stealth Rock, Toxic, Sticky Webs, Encore... and it has crazy high defensive stats so it can wall. You can also do weird things with Guard Swap, Power Swap, and Shell Smash Contrary, but those are too niche to go into detail to. While Shuckle definitely isn't the best, it isn't useless.
Now, as to why it doesn't learn Body Press. Shuckle is very light (45 pounds). Most Pokemon that learn Body Press are much, much heavier, so it makes logical sense as to why Shuckle cannot learn it -- it's too light. Or perhaps Game Freak didn't want to give it a tremendously powerful move (which isn't likely, they probably just forgot about it and we're rationalizing for nothing at all).