Ezt már régebben találtam valahol a Falcon-9-el kapcsolatban: There are many key points to this, probably none on their own sufficient to ditch the parachutes approach (except economics, those are good enough on their own), but together they make for a compelling case against it;
Descent control: As already mentioned, there's a significant guidance uncertainty with the use of a parachute system. Some of it comes from the weather in the lower atmosphere, and some from the geometry of booster stage and where the forces during descent are applied to it. Weather uncertainties mean that you'll have a wide and long best effort landing ellipse where the stage could be retrieved, and with slightly unfavorable weather downrange, further limits your launch opportunities. This might be excusable for NASA's human spaceflight endeavors (I personally don't share that opinion, STS could barely launch at all due to all the safety constraints imposed on it, and such parameters don't seem to stop Russians so much from launching). The other problem is that you can't correct for any such drift as easily with propulsive system due to stage geometry.
Structural stability: Geometry of the stage, being a long cylinder with its center of gravity low and close to the engines with only a certain percentage of propellants remaining, means that parachutes will apply force on the stage at the opposite end of it while its descent rate is reduced. This is not optimal. Deploying parachutes is also rather violent in terms of achievable maximum g-forces even during staged parachute deployment. This could introduce significant lateral forces to the stage if the parachutes deployed while the stage's long axis isn't aligned with its velocity vector. Rocket stages are not constructed to withstand much lateral force so they can remain light, and most forces associated with their launch and during staging are along their long axis, not lateral.
System complexity and reliability: Adding an additional, complex system of pilot, drogue and main chutes adds to the overall complexity of the already complex system, increasing chances of something going wrong. Reliability of parachute descent systems and their final design and reefing profile, is also hard to establish without first sacrificing a great deal worth of flight hardware. This is a rather tentative process with many uncertainties, and unless you have long years of testing and development behind it (like, say, NASA's Orion that has nearly 60 years long legacy in Apollo program), and can design your vehicle to withstand structural loads, it's not very likely that you'll get it right for an entirely different system before your budget runs out.
Weight (argumentative): Depending on parachutes wouldn't save much weight, if any. It's a complex system with many components, including (but not limited to) parachute fairing that would protect it during ascent, staging and hypersonic part of the descent, cabling for pilot, rogue and main chutes and a system to cut the lines when needed, canopies themselves and many additional sensors and actuators needed to make it happen. The stage would also require structural reinforcements to provide parachute attach points and withstand forces they'd introduce to the descent profile. Using parachutes also doesn't get rid of de-orbit, de-spin / stage rotation, and final landing cushioning retrofires. Such system might also require additional inflatable airbags to cushion forces associated with landing. Keeping some 10% of propellants in your tanks for the final landing burn and slowing down from stage's terminal velocity to near zero vertical speed seems pretty close to the added weight a parachute descent system would introduce.
Manufacturing volume: SpaceX is keen on lowering price of launching to space wherever possible. Serializing manufacturing processes while keeping them flexible and easy to implement future improvements (often by introducing innovative manufacturing techniques such as additive manufacturing, aka 3D printing) is key to keeping non-operational costs down. So while they offer to their customers an option - fly on expendable, more capable hardware and pay more, or fly on reusable, slightly less capable hardware and pay less - they do this, or rather - will do, with exactly the same parts as far as manufacturing is concerned. There are but minor differences between F9E and F9R, most of them modular, such as addition of the grid fins and landing legs to otherwise same, or nearly same stage. Using parachutes on F9R and no parachutes on F9E would go against such doctrine and require two different stages, one reinforced to support parachutes, and an expendable, lighter version that wouldn't require that. So you'd have two manufacturing lines instead of one. Not economical.
And there are some other, finer points that I'll omit to keep the length of this answer down. But I believe we can agree that going for a system that's already an essential part of the stage was the better option.
There's another way of looking at it, tho. Reusable hardware has one inherent property that it is worthless unless proven to still work. So, if you fail at recovering the stage due to, say, descent engine failure, and there's not enough time to compensate with the other of the nine Merlin 1D engines because they also didn't fire in time, then most of the value of your hardware you were trying to recover is near worthless anyway. How exactly will SpaceX be discarding reusable hardware once it's tested to no longer be suitable for reuse is still somewhat a mystery, but failure to recover the stage would be obviously one of the ways to do that.
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