My first thought was: "Because they're SO freakin' COOL!!" rofl
Honestly, I think because the Mustangs have departed from the original style for such a long time, people have almost "forgiven" Ford for the departure from standard Mustang doctrine with our models. Partially because of the Mustang II, Fox-body, and later models being so much more of a departure than merely stretching, lowering, and widening, as our models did. But I would also figure, and this is a parallel for our cars as well, the more current Mustangs have been taking aggressive-looking styling cues from Shelby and incorporating them into their concurrent models.
Case(s) in point:
- 2010 Shelby Super Snakes come up with inboard projector lights w/outboard parking indicators/signal "slots," when the base model went with outboard 'normal' lights and inboard parking indicator/signal "slots." People love it or hate it (I love it, personally).
- 1967 & '68 Shelbys have full-size driving lights in the grille opening (placement varied by application) - Ford likes that look, discontinues the Rally Lamp package and incorporates full-size high-beams in the grille opening (one of their better ideas, IMHO - it just looks mean).
- Shelbys come with woodgrain inserts, Ford follows suit. (I'm not much of a fan of that, but most people liked it I'm sure)
- 1971 Mustangs went with the nose treatment and NACA ducts ala 1969 Shelby styling cues - which was a good thing, if you ask me.
I'm torn with the change of direction, only so far as to say that I'm a big fan of the '69-'70 Mustangs... and equally as much the respective year Shelbys. But, transitioning to our model year designs was merely a natural progression, as "low, sleek, and wide" was the trend for sports/performance cars of the time. Not to mention, spreading out the shock towers to stuff in the big blocks was simply 'staying in the game' with the muscle car wars... which unfortunately fell off during the run of our model years.
The current sense of nostalgia for the older muscle car-era best-sellers has been somewhat realized by the manufacturers bringing back major styling throwbacks (Charger, Challenger, Camaro, Mustang, Nissan Z-cars, etc.)... but, it looks like it's time to move on again - hence, the 2015 Mustang design. If it's successful, look for the others to soon follow suit and stretch, twist, contort, and otherwise "ruin" the looks of their throwback model designs.
Just my observations. Don't know how close to reality they are, but they make sense to me.