Not just coffee...
It is not just coffee that blocks the drains...
When the Cabin crew pour milk and orange juice down the drain (especially pulpy juice) the milk curdles and you are left with a sludge the consistency of yogurt. This sludge does not flow too well...
The drains are not the best designs either, they are often too small a diameter, poorly routed and difficult to access and clean. There are many low points where sludge and gunk accumulate and block the drains...