Probably for the same reasons Carter lead Reagan by 30 percentage points and more at the same point in 1980. People aren't really focusing on general election match-ups and giving serious consideration to policy differences at this point.
What Really Happened in the 1980 Presidential Campaign
According to your thinking, Reagan should have been dropped from consideration during early 1980.
And people look at this national polling and forget that it is the Electoral College where the election is won or lost (#electoralvotesmatter....hahaha!). Only the polls of VA, FL, OH, NV, NC, WI, IA, CO, NM, MO and maybe a couple of others will be worth discussing.