Because it removes the technical challenge of modifying and manipulating airflow to the fullest degree. These rules only pose one challenge - KERS - and that challenge is laughably easy, since 2009-spec F1 KERS is neutered and restricted to death. A few teams will struggle, but by Melbourne, most of the teams (Toyota, for example, not) will have KERS running at nearly the maximum allowed power. BMW and McLaren already reached near-peak performance in the last test.
Effectively, the 2009 rules mandate a car that's even less complicated than the late-'90s cars. The simple fact is that in F1, simpler is backwards. Bodywork, with the tangential curve radius rule, is effectively "locked in" and there's barely room to maneuver. 50% of the front wing is spec-mandated, and the rest is highly restricted as well. The rear wing received additional restrictions, and the diffusers are now forced to be simpler. Engines can't get improved much (if at all), gearboxes have reached their peak (can't shift faster than no time at all, right? And the minimum weight is mandated, and maximum gears has been reached), tyres are spec-supplied, ECUs and electronics are regulated, and there's not much to improve with the brakes, either (they're already nearly fade-free, weigh next to nothing, and produce more force than the tyres can handle) - so what remains?
To this point, we had open areas where teams could truly innovate. Even if we consider just the cars of the last decade, which were already very restricted:
First we had engines rising in power and revs, producing a mind-boggling 1000hp from the 3l V10s, then 780hp (and touching 800) in the 2.4l V8s. Now what? After restricting them to 19kRPM and nearly no changes, they're now further regulated to a puny 18kRPM, and will be replaced by spec-engines - perhaps as early as next season.
Aerodynamics was an open field until now. While we had restrictions on dimensions, number of elements, "forbidden areas" and the likes, engineers were open to introduce new ideas. New ideas were implemented every season: From the curved new sidepods of the F2003GA and the unfortunate MP4/18, Renault's R25 cooling-gills, McLaren's bridge-wing, Ferrari's nose-hole, BMW's sidepod-shield connectors, and dozens of other new parts and ideas. Now, however, it's all pretty much dead: No special surfaces, specified dimensions and locations for everything, and only loopholes remain.
KERS is so tightly regulated and simplified that it won't provide a real challenge for the engineers after this winter break. By regulating maximum power, burst-length and energy stored, they've made it a neutered form of propulsion that will, at best, provide an extra two tenths - considering the extra weight at an unfavorable location (the Bridgestone tyres, and their slicks especially, perform better with a frontward weight bias - and KERS is heavy and in the rear).
Just thought I'd through in an extra does of subjective there yano 
