Why do fiber-optic interfaces have much higher bandwidth than optocouplers?
There is really nothing preventing someone from designing an optoisolator to go that fast. Fiber interfaces are generally designed for long range communications and so they have a very different set of design constraints. Fiber interfaces can use lasers or VCSELs and possibly separate high bandwidth modulators and wavelength division multiplexing to cram huge amounts of data into a fiber and transmit it several km. Optoisolators, on the other hand, generally have to transmit low frequency signals very short distances while being very small, cheap, and robust. It's not worth the extra expense to design very sensitive devices that can transmit multiple Gbit/sec if they are only going to be running at a few kHz. There really aren't a great deal of applications for that kind of isolation and that kind of bandwidth that don't also involve sending signals at least a few meters where common telecom or datacom transceivers can be used instead.

As far as optical transceivers are concerned, just getting the data to the transceiver at those speeds is not easy.  Right now the highest rate for a single wavelength that's commonly available is 25 Gbit/sec.  That's about 13 GHz of bandwidth.  You can't send that very far on a PCB as FR4 (fiberglass substrate) is quite lossy at those frequencies.  The SERDES modules on the chips to send data at that rate take up quite a bit of space and consume a not-insignificant amount of power.  The links are all current mode logic that have very small voltage swings of less than 100 mV.  The VCSEL or laser at the other end has to be designed to have enough bandwidth so that it can be modulated at that frequency, which is not so easy to do.  The lasers also consume significant power and dissipate most of it as heat, requiring a lot of cooling.  If you use wavelength division multiplexing, then you may need to use active temperature control as well, which uses even more power and generates even more heat.  All of this is expensive and requires a lot of engineering and process controls.  It also takes up quite a bit of space.  

Optoisolators, on the other hand, are a phototransistor and LED inside a glob of transparent epoxy.