Having been a rather important part of the IR staff, I feel I can speek for atleast the IR Staff if not most of IR.
The MPC world is great. I loved all the time that I spent in the Med scene with rivalrys back in the day with Dv, Carnage, Cp, DP, and then later on Chaotic. Back in the 07-09 range all med clans pulled the same 30-45 every weekend and there were 3-4 clans at all times who could win the day. Then around summer of 09 with Carnage,Dv and IR all pulling 70+ to fullouts it changed the scene and lead to a MPC world dominated by 1 clan at a time. Once it was a 1 clan show in the MPC world it all started to go down hill.
Also another huge facor was as somone said earlier; age. Most of the MPC were lead by a very small core staff of 1-5 people. In IR atleast, Myself, Junior, Zo, Tal, Owen, Will Turb, and many others had been either leading or atleast a part of IR since our younger days (11-14 range). As the clan grew up so did we. I mean I am 17 now and in my junior year. I don't have the time to balance a life with leading a runescape clan. I mean when it comes down to it, this game sucks. Back to the point when all/most of the staff get to that critical quitting age the clans fall to **** only to open up the community to the next generation.
RS Pure clans have scene 4 Era's
1. Old Wilderness
2. BH crater/Clan Wars
3. PVP
4. Return of Old Wilderness
In each of these respective eras many of clans have been created, flurished and then died. Few clans such as FOE,EOP,FI,MM,TLP, CP have been able to do well in all 4 eras. Majority of clans have lived during one or two eras and died off after a while only to be replaced by a new clan. It is up to a MPC to put in the time and effort needed to dominate the MPC world in this new 4th era.
In general, MPC's of today need to learn to deal with slumps. Many of these clans and IR can be acused of this aswell, have closed like 2-5 times. Stability is whats needed to keep MPC alive.
*Note I refuse to spell check this as I am trying to write an essay, troll my flawed grammar and spelling all you want =)