In the past, when it was almost completely about drugs, their activities were concentrated in two types of area:

• transit corridors (coasts, ports and US border)
• growing areas (e.g. poppy fields in the mountains of west, like Michoacán, Guerrero)

That left a strip up the centre less affected (e.g. Puebla, Edomex, Queretaro).

Now that they've leveraged decades of drug money to expand into other enterprises (e.g. extortion), they've expanded into other geographical areas too. But the remains of that central strip can still be seen on the map with white & whitish areas.