MOTUC was never in retail, other than the select few 2-packs.
1) they'd look outdated compared to today's toys
2) it wouldn't make sense after MOTUC to reduce quality, appearance, etc
2) there is no media to support & promote the line (no tv show, no film, etc)
Unless there's a new show, or unless there's a film, there is no reason for Mattel to go full retail with MOTU.