Why aren't all crystal lattices 6:6 or 8:8?

Lets start off with a reminder why NaCl is 6:6 and compare thaty to CaCl2

The NaCl the structure is 6:6 because in that lattice the structure the ions take up is controlled by the size of the smaller Na+ ion and how many Cl- ions you can fit around it, (i .e. Na+ is small so you can only get 6 Cl- around the Na+).

The other number 6 (in 6:6) comes from the fact that to maintain the 1:1 ratio that NaCl must have, you need 6 Na+ around each Cl- too. You could get a lot more Na+ around the Cl- but that would spoil the 1:1 ratio.

(Why must NaCl be 1:1?

Sodium has 1 electron in the outer, which it needs to lose to form a stable ion. Chlorine needs to gain one to from a stable ion. So 1 sodium needs to lose 1 electron to 1 chlorine to be stable. Happy Days.)

Now, lets apply the same logic to CaCl2. Whatever structure CaCl2 takes, it must maintain the 1:2 ratio (for the same reason as above).

The 6:6 structure maintains a 1:1 ratio so it would never work for CaCl2.

You don't need to know the structure CaCl2 takes, all you need to know is, it is not 1:1, so can't be 6:6 (i.e. the same as NaCl.)