To be fair the mount achievement system was new to the game then, killing the bosses in strange ways was something new to many people and we were too busy with other stuff that we weren't even thinking about it at the time.
It was not actually hard to get the T7 achievement drakes though, many of the achievements were nothing special and the main stumbling block for most people was simply not really being aware of the achievement at all or not being able to pull off Immortal.. Clearing naxx was easy, clearing naxx without anyone dying was something that a single mistake from a single individual would make impossible and I'm fairly certain a lot of guilds had a couple of players who would make this impossible.
Some of the achievements were stupid too, like undermanning bosses as an achievement resulting in purposely having to sit players for a raid. We first caught on and made an effort in my guild for this in Ulduar and pulled it off in time only to find Blizzard were no longer removing them anyway, which was slightly annoying at the time especially given that the Ulduar meta was a lot more challenging than the Naxx.