Funny thing is I can't remember any big debate when 7-cell packs went by the way side. If memory serves the rules at that time stated 7 cell max. People just started running 6 cells.
7 cells were used when they had lower capacity cells (1200-1700 mah). You couldn't go to a lower wind without dumping the battery before the race was over. When higher capactiy cells came out 1700mah+, you could run a lower wind and and still make runtime and have the same speed with 6 cells.
Technology went from using 16-18 turn motors with 7 cells in 2wd to 6 cells with 12 turn motors in 2wd.