The cassettes commonly used for MTB riding have larger low cogs than cassettes used for road riding (32 to 36 tooth versus 28 tooth and smaller). Cassettes on MTBs also have wider ranges than what we see and use on road bikes; an 11-32 cassette used in conjunction with your 50/39/30 crankset technically requires a RD that can handle a chainwrap capacity 41, whereas your 11-26 cassette with that same crank only requires a capacity of 35.

MTB RDs handle the extra chain required by having a longer cage. If you put a "long cage" road RD next to a "long cage" MTB RD, you can see that the MTB RD cage is noticeably longer. The amount of chain, when not needed in some gear combinations, has to have somewhere to go; in this case, the longer cage holds some of the extra chain in the greater distance between the two RD pulleys, while yet more of the extra chain resides in the lower run of the chain loop...which has also been made longer by the longer dimension of the RD cage.

Think of it this way: In a world of limited choices, the RD manufacturer could offer us one type of RD that would function for MTB and road use, and that RD would be a long-cage MTB derailleur. It has the chainwrap capacity to handle up to 45t, but has no lower limit. But road cyclists want to use components that are light and offer snappier shifting. The result is a road RD that has the smallest dimensions possible that can still perform well with 27t and 28t max cogs and narrower range cassettes.

Sorry. I only make it sound complicated.