i) Some scholars (e.g. Evans, Edwards, Gundry, Stein, Wright) think the original ending was lost. I myself don't think there's any compelling reason to believe the ending we have (16:1-8) is not the original ending.
To be sure, Mark's account gives the Resurrection somewhat short-shrift compared to Matthew and Luke. However, that's a misleading comparison. After all, his Gospel is much briefer to begin with.
ii) More to the point, the ending may be briefer simply because Mark had less information than Matthew, Luke, and John. He may not have seen the risen Christ, unlike Matthew and John. Likewise, he may not have interviewed as many people as Luke. He just wrote what he knew, based on personal observation, without conducting the kind of extensive investigations that Luke did.
To take a comparison, consider the Civil War. You have foot-soldiers writing home. Their accounts are based on personal experience. What they saw. It may include anecdotes from comrades.
Then you have accounts written by the generals. That's based in part on firsthand observation. But they also have a much broader sense of the war. They are in communication with other commanders. They know what's going on elsewhere.
Then you have Civl War historians. They comb through a wide range for primary source material. They didn't witness the events, but their account is based on eyewitness material. And it covers much more ground precisely because they aren't confined to their personal experience.
On this analogy, Mark is like letters by a foot-soldier in the Civil War. He writes about what he's seen in theater, as well as information he's garnered from campfire conversations with his comrades.
Matthew and John are like Civil War generals. They write from firsthand experience as well. But they are privy to things the foot-soldier is not.
Luke is like a Civil War historian, who collects oral histories from veterans.
iii) But assuming, for the sake of argument, that the ending is less than we'd expect, here's another possibility: what if Mark saw that he was running out of room on his scroll, and had to present an abbreviated account of the Resurrection?
It's usually said that Luke's Gospel pushes the limits of a scroll. Admitted, Mark is much shorter, but was there a standard length scroll back in the 1C? It's not like you had mass production.
For that matter, I assume longer scrolls were more expensive. What if Mark couldn't afford a full-length scroll, so he had to skimp a bit towards the end? It's hard to estimate how much space you will need to say what you want.