Positivism does not conceive knowledge as an activity: to positivists, the object of their study tells them what they need to know, thence they are stuck in contemplation of reality. As Marx pointedly remarks,
The chief defect of all hitherto existing materialism – that of
Feuerbach included – is that the thing, reality, sensuousness, is
conceived only in the form of the object or of contemplation, but not
as sensuous human activity, practice, not subjectively.
This would be the case of positivism, either in its Comtian semi-religious approach or in its more common sense in the writings of Mill, Spencer, etc. Conversely, to Marx, knowledge is an active (thence "dialectical") process. One view is irreducible to the other, even if there are superficial similarities.