Because attunements were an active detriment to the game, they locked a huge part of the player base out of end-game content for essentially no reason. They were acquired without effort for people who had been raiding since the start of expansion, but were nigh impossible to attain for people who weren’t.
They made recruiting impossible for guilds since having to spend a whole week of raiding to attune a single new player was a very hard sell to the existing raid team. What actually happened is that raiding guilds would simply cannibilise already attuned players from lesser progressed guilds, and those lesser progressed guilds simply died out.
For a huge segment of the player base in TBC, raiding meant Kara clears every week, and maybe Gruul PUGs, until such time as attunements were removed and they could apply to actual raiding guilds.