Just some thoughts: As some have mentioned, there is evidence for early references to the Majority Text (with the longer ending of Mark.) I appreciate that, and I happen to agree that the longer ending does belong there. However, just because a source is early doesn't make it accurate - I'm referring to Vaticanus and Sinaiticus, which are called the "oldest and best" manuscripts, primarily "best" because they're early. We don't have the original manuscripts. We have copies of copies and copies of copies of copies. You could have an "early" manuscript that is the twenty-third generation copy, and a manuscript that is a couple hundred years later, that is only an eleventh generation copy. In this case the "early" manuscript could contain many more errors than the later manuscript. Therefore, being an early manuscript alone, is not enough to discredit later manuscripts.
Also, there are a few other possibilities. What if the Majority Texts (the later manuscripts of which most, if not all, contain the longer ending of Mark), were actually present in the first and second centuries, but they were in such popular demand that they were destroyed by such great use? They were simply worn out. This could explain why no older Majority Texts (with the longer ending) are found. I don't believe the argument "There are so many of those manuscripts; surely they could have found one or two," would be valid. I wouldn't believe the Majority Texts started out with such huge numbers. They would have started small and grown over time.
Along those lines, look at the climate. Vaticanus and Sinaiticus (again - the "best" manuscripts) came from Alexandria, Egypt, an ideal climate which would preserve manuscripts beautifully. Look at the location of most of the Majority Texts - Asia Minor, with a climate that would destroy manuscripts upon much exposure. This could also explain the lack of the early Majority texts.
Speaking of location, there's something to be said about the Vaticanus and Sinaiticus coming from Alexandria, Egypt, while the Majority Texts come primarily from an area where the original Manuscripts were written/written to, and New Testament Churches were planted and influencing their society. I would think the likelihood of more trustworthy manuscripts would be greater from those in close vicinity to areas where the original manuscripts had been present.
The number of manuscripts has already been discussed in this thread, but I think it's worth a mention, because there is a reason for the huge number of manuscripts with the long ending of Mark. And again, we're not talking about but a very small number of manuscripts that are supposedly the best, on which we're putting such a high priority, and most Bibles today are based on these few manuscripts. Disclaimer - I'm not a KJVO/NKJO proponent.
Lastly, although this doesn't specifically mean what we have in vss. 9-20 are the final verses to end the Book of Mark, we can almost be assured that Mark 16:8 is not the ending to the book. "So they went out quickly and fled from the tomb, for they trembled and were amazed. And they said nothing to anyone, for they were afraid." NKJV The very last line of the book being "And they said nothing to anyone, for they were afraid."???? I think not.