READ and be less Ignorant.
For same reason I combine every raid in classic and TBC. When TOTC was released people didn't drop Ulduar (Like they do it now. New tier released = prevous tier dead). Naxx was too easy and farmed to death and when Ulduar was released everyone indeed switched to Ulduar and Naxx was basicly dead. ICC tier (both raid and dungeons) were so easy and well rewarding that 1-2 weeks after release Ulduar and TOTC were dead. (That's what I remeber from my server. Your personal experience may be different of course)
That's a reason for Ulduar lasting 238 days however TOTC was released just 112 days after Ulduar. It was still relevant content.
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If you were a casual raider SWP meant nothing to you and if you were a heroic raider naxx bored you to death. They do a much better job of matching content to multiple player bases these days they just need to find a way to avoid content lulls going forward(or at a minimum reduce the lull).
In all fairness, RS wasn't really a tier after ICC
916 Mistweaver
909 Holy Paladin
To be fair, you have to divide days by number of bosses to see the true length of a tier, SoO length looks fine that way, Sunwell beats all.
dunno whats yout definition of "strictly spearated tiers", but they are heavily overlapping ilevels now just as much as they did before. I expected you to merge, like ZG into BWL tier or OS into ICC tier, instead, you did the very same thing as OP - biased propaganda BS. also lol at merging Ulduar and ToC into a single tier...