As others mentioned. Because he has no idea how he wants his team to play, doesnt have any idea what talent England has , limits his selections pretty much to the usual suspect teams and has underachieved with the world class talent he has.
His main strength as @kent_villa noted is he handles the press ok. Which is a strength in a national job I admit.
But flip it, if Smith left Villa tomorrow and you wanted to stay in the Premier League this season and you had your pick of Premeier League managers & Southgate how high up would Southgate be in your interview list?
Because for me he would be 19th, maybe 20th.