Any ground meat will create a meatloaf, but for a truly exceptional dish we must consider the muscle structures of meat, the fat content, and how the proteins constrict during cooking.
Beef comes from large, heavy, animals who walk around all day requiring a lot of effort from their muscles. This creates a tighter muscle structure with fat that is hard at room temperature, and rich flavorful meat. So, a beef only meatloaf tends to produce a loaf that is more coarse, drier, and has a less subtle flavor.
Pigs are much smaller, spend most of their time laying around, and thus create a more delicate lighter meat with a looser muscle structure and softer fat. This is the reason we use pork, and pork fat, for making sausages. An all-pork loaf then tends to be much tighter, less flavorful, but moister than an all-beef loaf.
The trick is to find the right ratio of beef to pork. This will help you create a loaf that has the perfect combination of rich meaty flavor and tender flavorful fat.