I usually use 1 egg per pound of meat, no less than 1 egg per 1.5 pounds. The protein in the egg acts as a binder.
Also depends on how much fat is in your meat and how you let the fat drain from the meatloaf as it cooks. The breadcrumbs will absorb some fat, but the rest needs to go someplace.
Also, if you slice the meatloaf right out of the oven/cooker, it will be more crumbly than if you let it rest for 15 or 20 minutes.
-gbeck