Apoc - I figure it is getting kind of difficult or expensive to get the parts to make the Deathwheels. I figure this would be a reasonable alternative to having to go scrounge parts. It is a relatively simple design, so the cost shouldn't be all that high. The downside is that all the Deathwheels in your army would look the same - not very Chaotic.
GlynG - You're trying to apply logic to a game 38,000 years in the future, where daemons, magic, and science co-exist. Stop it.

While I agree that the Deathwheel in its current form is kind of silly looking, I'm not too keen on modifying a design that the community, for the most part, has accepted. If I go outside the accepted parameters of size, we start running into issues of advantage/disadvantage due to model size. If you have two Chaos armies fighting each other, one guy with the current Deathwheels, and one with something designed differently, there could be eyebrows raised. Granted, Epic players are far more lenient than 40K players, but the point still stands.
Personally, the description of the Kaban Machine in
Mechanicus was how I envisioned a Deathwheel (or at least a Deathwheel proxy).