In the last expansion, Sylvanas was an extremely controversial and ruthless Warchief -- but many players, especially from the Horde, still sympathized and supported her regardless.
Even after she left Orgrimmar, many Banshee Loyalists remain loyal to her cause. Even today, many players still believe she has some greater motives at play -- or that she may possibly somehow redeem herself, possibly by improving the Shadowlands or ending the flaws of the Pantheon of Death once and for all. If you given the choice to support Sylvanas, would most players actually be willing to support her?