Blizzard recently made a poll asking what your favorite game mode is:
41% of respondents prefer a game mode that involves Classic. 13% specifically prefer hardcore. 11% specifically prefer Classic Era.
Whether you go on Twitch in the middle of the week or weekend, the top 2 streamers for the WoW category are specifically playing hardcore. More than one third of all WoW viewers are watching a streamer play hardcore.
A pretty significant number of players are interested in Classic over the new content and game play that is being released in Retail. My guess is that these players are more interested in the old game play than the old content. Hardcore has added something fresh to that game play.