Because some prefer the SE Pro. Depth of both machines is pretty much an even match because they are using the same coil (and that's where any real advancement in depth is done these days on these VLF machines, especially Minelabs which are maxed out in VLF technology. In fact, I don't see any difference in depth between my friend's Etrac an my GT with 12x10 on undug targets...or in unmasking ability.) It's the coil that's the key, and since this new machine uses the same 11" Pro Coil from the looks of it I bet it won't be pushing depths any further.
The main difference is the SE, like the old Explorers, has better audio on silver at depth with the classic Explorer "flutey" tone on them, while the Etrac is said to have a bit better VDI at depth. Some prefer one of those over the other. Also, some (including me) don't like the way the Etrac's VDI system is set up versus how it is on the Explorer. I have a friend who owned an Etrac for a while and even made some real nice finds with it, but went back to the SE Pro. I've got two other friends who both own Etracs. One of these guys used to have an older XS Explorer but he likes the Etrac better. Different strokes for different folks. If it was my choice, having used an Etrac here and there and owned Explorers in the past, I'd opt for an SE Pro over the Etrac myself. But that's me.