I'd like to speak of this section not in comparison to Luke but by comparing it to the rest of Mark.
Mark 6:45-8:26 has a number of peculiarities which set it apart from the rest of the Gospel.
1)This section of the gospel contains a number of 'doublets' of episodes in other portions of Mark
6:45-54 -Walking on water, a form is found in Mark 4:35-41
8:1-10 -The feeding of the 4 thousand, Mark 6:30-44.
8:22-26 - Healing of the blind man, Mark 10:46-52
2)Two of the healing stories in this section occur with Jesus going through elaborate almost ritualistic actions: 7:32-36 and 8:22-26. Nowhere else in any of the gospels can one find such behavior by Jesus when there is a healing miracle.
3) The "Mark's" vocabulary in this section of the gospel exhibits some features primarily found only here. For example:
---The adjective "without insight" is only used in this section.
---"to understand" is used 4 times in this section and only once outside of it; and only with regards to an allusion to a biblical passage (Isa 6:9-10).
4) This section details episodes of Jesus' ministry which occurred outside of Galilee.
These points are meant to show that this section of Mark was added after the gospel was completed and not by the original author; but by someone who attempted to mimic the style and vocabulary of the primary author. This person was ultimately very successful, for the differences in this part of the gospel although easily discovered, can have other valid explanations besides additions by a second author.
But if the theory is valid and the entire section is an addition, it would explain why it is missing from Matthew and Luke; for it was added after they used the Gospel of Mark in writing their own.
How can one explain the first version of Mark being completely lost and only the second version surviving the millennia?
How does one explain all the surviving versions of John having chapter 21 when internally, it is clear that the original work ended at chapter 20?
Time can pass between authorship and public release of the work.
Clement (c. 195) stated that the gospels with genealogies (i.e., Matthew and Luke) were "openly published", before mark. This May mean that Mark's Gospel; although written first, was initially in private circulation.
Something that may back up this hypothesis is, Clement's Letter to Theodoros regarding a more spirital, expanded version of Mark. If it is to be considered genuine; this gospel, stored in Alexandria (Where tradition states Mark went on to preach and become it's first Bishop) was a version of Mark that is not the version that has come down to us. I'm not here to weight the pros and cons regarding this unknown version's authenticity. I only bring it up to show that the history of canonical Mark may be a very complex one.
In closing, this is not my theory, for a more complete presentation of the theory see Helmut Koester's Ancient Christian Gospels pages 284-286.