It is more difficult to realize why Mark is anti-marcionite, also.
The feature of Mark is his secrecy, his enigmaticity.
Therefore, if Mark is so enigmatic, allegorical, esoteric and secret, then the his opposition to Marcion's Gospel (assuming that Mcn is the Earliest Gospel) can be explained only by the his being a gospel enigmatic, allegorical, esoteric and secret.
The contrary of esoterism is a public revelation of a new message.
Surely the marcionite Jesus does not make mystery at all of his deity, the deity of an Alien God, or of the his new message (that is the itself ot the his identity).
An example may explain why precisely just the secrecy and the esoterism are tools against Marcion in Mark:
when a Christian apologist (at least in my eyes) - Morton Smith - found (or invented) the so-called Secret Gospel of Mark, the natural reaction of the stupid hoi polloi was the following:
1) ''this Secret Mark is a Esoteric Gospel'',
2) ''therefore'', it has to be more old than Mark itself.
In other terms, secrecy would be ''evidence'' of greater antiquity and authenticity, in the eyes of the stupid hoi polloi.
Yesterday as today.
Another example:
Paul claimed that he received a special secret revelation by Jesus himself.
This claim, in the eyes of the stupid hoi polloi, means that Paul used the secrecy of the his revealed Gospel as a tool against the public Gospel of the previous Pillars, implying so that the latter didn't see and didn't know something that Paul knew and saw (the true ''celestial Jesus'').
In other terms, the secrecy of Paul's Gospel would be ''evidence'' of a greater role for Paul than the role for the Pillars.
Another example:
the Gnostics called themselves ''Gnostics'' because they knew the ''secret things'' of the reality. Obviously as a counter-cultural tool against the previous traditions and common knowledge.
CONCLUSION
therefore, when Mark added Mark 4:11-12 in the his version of the Gospel -- as I have explained here -- he is doing so to claim greater authenticity and antiquity than the Gospel written by Marcion.
The logic would be: My Gospel is esoteric ''therefore'' it is more old than your Gospel.
It is surely a fallacy. Secrecy, symbolism and allegorism is really another mere tool of theological propaganda.
As corollary:
FACT 1: the author of the Earliest Gospel was a very known personage.
FACT 2: the Earliest Gospel was a Public Gospel from the first day of the his publication.
FACT 3: Paul-Louis Couchoud was basically right:
(Creation of Christ, p. 399)The god of the Jews said, Aure audietis et non audietis (Is. vi. 9). Jesus, on the other hand, wishes all ears to be opened (T. iv. 19). All should listen, since there is no longer anything hidden; everything is made clear.