I think Alan and Mel are popular, and I would suggest that they are more popular than Len Goodman.
Unfortunately, the powers that be fail to appreciate that it is not the 'personality', it's the personality in the format that works.
Len Goodman in Strictly came over as an avuncular and knowledgeable gent with an element of cockney charm, and he was limited in terms of time and exposure.
As usual, the TV bosses made the fatal mistake of assuming that the appeal was him personally, not him in that context, so they gave him a quiz show and some radio shows on his own and, hey presto ...
Without the cushion of the successful format, and lengthy exposure to Len on his own, he is discovered to be a one-trick pony, a rather boring and colourless individual whose 'cockernee' charm wilts amazingly quickly after ten minutes or so of constant exposure to it.
Cue a fade into the background, and the phone not ringing to say Paul O'Grady needs a stand-in.