As you move off axis you move further away from one driver then the other, in other words the distance between you and each of the drivers is no longer equal.
Now when the driver are close together (small CTC) the change in distance is less then if the drivers are further apart. At an extreme picture being 90 degrees above or below the speaker and measuring your distance to each driver.
That change in distance as you move off axis creates problems, the sound from one of those drivers now reaches you before the other. Imagine the sound from the two drivers at the crossover frequncy being two sinewaves perfectly in sync (in phase) while you sit on axis, as you start to move off axis the sine wave of one starts to move ahead of the other and they are no longer in sync (phase). When they start to move out of phase they no longer sum perfectly, 90 degrees out of phase they don't sum at all, more then 90 and they start to cancel each other. Move far enough off axis and the sine waves will reach a point where they are completely out of phase (180 degrees) and you get a complete cancellation at that point.
That all relates back to your distances between the drivers and your crossover frequency. If your drivers CTC distance is 1/2 the wavelength of your crossover frequency you will see the cancelation 90 degrees off axis above and below. The futher apart the CTC distance the narrower that window (forward lobe) becomes, smaller CTC wider lobe. It is very hard to get a CTC distance of 1/2 crossover frequency wavelength, its only like 6.8" at 1000hz so you can see how this becomes a problem.
As an example here is what happens to the frequency response when moving off axis (this is 0 to 20 degress below on my MTG-08):
Edit: The MTM still causes problems becuase the woofers/mids will cancel with themselves at the crossover frequency as you move off axis before they cancel with the tweeter since they now have the furthest CTC distance.