Wizz may have made their London area airport decision some time ago with Luton but they do now operate from London Gatwick albeit just the one route to Budapest.
Although I can't see Southend being added anytime soon, with the UK about to leave the EU Wizzair current traffic will more than likely decrease to a certain extent so I can't see them adding another London airport let alone Southend where as Luton totally fulfils there needs.