It is some time ago that we had a rather interesting thread about the Great Valley and why it would be a saver place than any other valley.
The only reason explicitly stated in LBT 2, the Great Wall, doesn't sound very plausible all by itself. Valleys are usually surrounded by mountains and it would be unlikely to consider the Great Valley the only place surrounded by mountains so steep and rugged as to allow entrance only at a few spots. Later sequels than LBT 2 (for example LBT 4, 7, 8) clearly showed that herds enter and leave the Great Valley. Those herds included longnecks, the largest dinosaurs we know of. There is no plausible reason why a sharptooth should not "fit" through a gap which allows a longneck. So the Great Valley cannot possibly be completely sealed of by the Great Wall alone.
My guess is that the reason for the relative safety of the Great Valley is much simpler.
Sharpteeth probably dread that place and only go there if they have to.
In the Great Valley the dinosaurs of all kind stick together and we saw that their sticking together made their defence of the Valley against Chomper's parents in LBT 2 rather effective. No sharptooth (excepting Chomper) who entered the Great Valley fared well. In LBT 6 we saw 3 sharpteeth being killed for entering the Great Valley or even its outskirts. It is possible to assume that in the long time of the Great Valley history those three sharpteeth were not the only ones who lost their lives for entering the Great Valley.
Now lets take a look at the Mysterious Beyond around the Great Valley as it was presented in LBT 2 and 3. What do we see? Corpses! Skeletons! A sharptooth's paradise!
Out in the Mysterious Beyond Sharpteeth may pursue migrating herds and go for the stragglers rather than facing a whole bunch of dinosaurs of all kinds who want to "stand their ground" by all means (while a herd in the inhospitable Mysterious Beyone would just want to get on to the next hospitable place as quickly as possible). Live is much less dangerous and probably more rewarding for a Sharptooth in the Mysterious Beyond.
Another indication that this assumption may be true is the fact that Chomper's parents left the Great Valley the moment they had recovered her son. Neither of them showed the slightest sign of any desire to stay in the Great Valley for any second longer than necessary.
I suppose the Great Valley is not a safe place because Sharpteeth couldn't go ther. I suppose the Great Valley is a safe place because no Sharpteeth in a healthy state of mind would want to go there.