The gospel of Mark is conventionally dated to somewhere very close to the Roman-Jewish War (Telford, 1999, p. 12ff). There are clear references to the 70 CE destruction of the Second Temple in Mark 11:12-23, 13:2, 13:28, 15:37-38, so there doesn't seem to be much doubt that it dates to at least as late as this period. But I can't seem to find any similarly solid argument as to why the date can't be several decades later.

Luke drew from Mark, so Mark has to be earlier than Luke, but Luke's date is very uncertain, possibly as late as 110 CE.

Brandon (1961) paints a pretty plausible picture in which the Markan author, living in Rome, does the final literary work on the gospel right around the time of a huge triumphal procession in Rome marking the fall of Jerusalem. It sounds reasonable, but not conclusive.

Brandon, in addition to his general plausibility argument, does give one other argument that seems to have more independent persuasiveness, which is that Mark 3:18 refers to Σίμωνα τὸν Καναναῖον (without giving a translation, as he normally does for transliterated Aramaic words) in order to avoid controversy about Jewish nationalism (which would have been a big deal ca. 70 CE), whereas Luke later reverts it to the more normal Greek Σίμωνα τὸν καλούμενον Ζηλωτὴν (Simon the Zealot). This sounds pretty good, but hardly enough of a solid argument, all by itself, on which to rule out dates much later than 70.

Is there some reason why it's really not plausible for Mark to be much later?

*References*

Brandon, S. G. F. (1961). The Date of the Markan Gospel. New Testament Studies, 7(02), 126. https://sci-hub.se/https://doi.org/10.1017/S0028688500005336 

Telford, 1999, The Theology of the Gospel of Mark