Usually when land is not part of a country, it is a disputed territory. In the case of Bir Tawil, it’s the opposite: no one wants it. Neither Egypt nor Sudan claim it. Since this is the only piece of land outside of Antarctica that no country claims, you could say that Bir Tawil is the most unwanted territory on Earth.
But why do both Egypt and Sudan refuse to claim Bir Tawil? Sure it’s small, but it’s land! (Take a look at the picture. Who wouldn't want that?) What kind of a country refuses to claim land? The answer lies in the nearby territory, the Hala’ib triangle. The Hala’ib triangle is another territory between Egypt and Sudan, but it’s ten times as big as Bir Tawil. Both territories are the result of the difference between two the historical boundaries between the two countries. The boundary established in 1899 leaves the Hala’ib triangle with Egypt and Bir Tawil with Sudan. The boundary established in 1902 does the reverse. Both countries claim the Hala’ib triangle, so each of them obviously prefers the boundary that gives it to them, and simultaneously gives Bir Tawil to the other country. Claiming Bir Tawil would be the same as claiming that you had no authority over the much larger Hala’ib triangle.