Because people want a smooth raid experience and want their time in raid respected. Among big guilds like Illusion, someone playing a non-meta spec is fine since they can 24 man every boss easily, but if you're in a guild actually progressing bosses, it's disrespectful to 24 other people in the raid to perform intentionally suboptimal when you're not farming all the content.
Demo lock buff is the same spell power buff as Flametongue Totem and it is significantly better, giving 650+ SP in BIS gear, compared to the 300 or so you would get from Flametongue Totem, so it's a complete waste of talent points while removing one of the best Restoration talents.
If you don't want your time in raid respected and don't mind people playing suboptimal specs during progression, then by all means make things more challenging for yourself and the other 24 people in your raid, but not everyone wants this. Considering you can probably clear 25m HC 30% ICC with Naxx25 gear with an ideal comp, your comp at 5.8k+ GS doesn't really matter on Icecrown, and for the record, at least the people in this forum aren't saying this with condescension; it's simply just making a note of the differences of the realms. I would transfer all of my characters to Icecrown if I could, and this is coming from a Lordareon veteran since 2015 and I would recommend any new player starting on Warmane to play on Icecrown over Lorda.