Why are hdd cables failing?

There are a few different issues here. The older systems had a SATA II (3.0 Gb/s) drive (the logic board it self could support SATA III drives but at the time Apple couldn't get them reliably. Apple also didn't qualify a SATA III drive with the older cable only a SATA II drive. Which is the reason if you replace a SATA II drive in these systems you need to also replace the cable here as well. Don't forget to make sure the systems firmware is up to date as Apple did not enable SATA III in the chip set in the older firmware. Here's the link to what you need to be at or newer: About EFI and SMC firmware updates for Intel-based Mac computers. Today most people are using Mavericks or newer which also had a firmware updater in the installer so this is becoming less of an issue. The newer firmware also addresses Find My Mac services and some other things.

The other issue here is the routing of the cable in both 13" & 15" models. The issue is banging of the bottom lid can wear the cable as the compression of it over time breaks it down.

The cause is a bit different between the systems the mid plane black plastic clips (2) the cover snaps into (on the 15" models), also act as a spacer so when the lid depresses it protects the cable but if the clip is missing or damaged it then damages the cable. Here's an image where you can see it: Clip is just above the Red marked screw. If the one near the drive bay is broken swap it out with the other.

On the 13" models I've also started adding a piece of foam tape to the cover where the drives metal bar hits as well as on the battery near the logic board connection point to help lessen the impact. Here's an image of the drive bracket and the point of wear to the cable: HD bracket as well as the point the cable crosses over to the logic board over the optical drive by placing a piece of foam on the battery just to the right of the battery label in this image.

While both of these actions helps prevent future failures it doesn't fix the already damaged cable. Apple has improved both the frame areas in the newer models as well as thicken the cable a bit so it's a bit more durable. I strongly recommend putting in the newer version of cable in all of the systems. While in some its a bit tricky it still will fit.