Few people consider the time they've had to spend diagnosing and fixing a leaky keg, hand-scrubbing small corrosion spots inside the keg itself or tutoring a friend in keg operation. The supporting equipment for kegs is another matter altogether. Kegs can't be used without the hoses, taps, wrenches, seals, o-rings, valves, regulators, bottles and gases that go with them. Any of these parts that come in contact with beer need to be cleaned and cleaned well. Some of these items, such as beer faucets and quick-connects, need to be disassembled completely and cleaned on a regular basis.
Kegs require extra parts on hand to deal with routine wear and unforeseen equipment failures.
This translates into extra space and money. The keg will either leak, or you won't be able to get the beer out of it. Because all cappable bottles are interchangeable, one never gets into the position of requiring a specific kind of bottle from a specific manufacturer. Compare this to a particular poppet, o-ring or valve on a keg. The first time you need a part for your keg and don't have it, you'll start stockpiling parts, which requires space, time and money to track.