the odds are so stacked against "I'll have another"
the Belmont is just so much farther to run then the derby / preakness.
It takes a complete different horse to run that length.
If you notice, every year the field of horses have a great % of horses that run both of the first 2 legs.
But there is not alot of those horses who run the Belmont also. Sure some do, but it's just ALOT lower %