I wonder whether Pep is slightly concerned about his injury troubles in the past and is keen not to overplay him when he’s in such a rich vein of form.
Had presumed all 3 CBs would be rotated once Laporte was back but it seems to have been Dias + 1 other since then. IMO he has outperformed Dias in the last few games (only by a very fine margin - Dias has still been absolutely phenomenal but Stones has looked so assured and imperious recently).
That's the only reason I can think that he alternates games at the moment. The form he's in he should be the first choice centrehalf, but his injury record had been a big problem for him, so maybe one game a week is the wise option. Especially as the alternative is laporte... We're spoilt.