Well that's easy: Force Lightning is a ranged attack with zero cooldown in case of Madness, Ravage/Master Strike is a melee attack with a 27 sec cooldown (assuming everybody takes the Ravager skill). If someone interrupts Force Lightning, you can use it again in 4 seconds; if someone interrupts Master Strike, you can't use it for half a minute.
Having a talent that makes Force Lightning uninterruptable for a short time every minute or so would be okay; but just making it totally uninterruptable is way too much.
It is already designed around that though. Ravage is completely free, and costs zero resources. Marauders are a class that were intended to be limited by their force generation. The last single tick of ravage can easily hit for more than all 4 ticks of force lightning. If ravage was interrupted maras can easily use a rage generator if they are out of resources or if they have plenty of resources can do viscous slash at any time. As well as having better abilities than that depending on what tree they are specced, although those do have CD's. Madness sorcs that you mentioned have shock on a 6 second CD, crushing darkness, creeping terror, and deathfield on a 15 second CD, and affliction with no CD but only able to apply a single stack to a target which lasts 15 (21) seconds. So applying that does absolutely nothing. If somebody interrupts my force lightning right after I threw a shock out, there is absolutely nothing I can do DPS wise for 4 seconds. Static barrier and my self heal are also on long CD's and if those are on CD too, I can do exam you nothing. Maybe use an unspecced heal on myself or a friend. 3 seconds for 2000 HP heal...
Interrupts hurt us more than snipers or maras who have bunches of abilities. But both of them are un-interruptable while we are completely able to be interrupted and have the animations that make it the most obvious of any class that we should be interrupted.
Like I have said. Salt in the wound. And bad design.