Two reasons jump to mind.
First - time pressure. Shortly after Harry wakes up he is told they will be performing The Kiss "any moment now". Not a lot of time to convince the adults (against the word of Snape, and without Lupin, who is off being wolfy) to hold back.
"Ah, you're awake!" she said briskly. She placed the chocolate on
Harry's bedside table and began breaking it apart with a small hammer.
"How's Ron?" said Harry and Hermione together.
"He'll live, said Madam Pomfrey grimly. "As for you two you'll be
staying here until I'm satisfied you're Potter, what do you think
Harry was sitting up, putting his glasses back on, and picking up his
"I need to see the headmaster," he said.
"Potter," said Madam Pomfrey soothingly, "it's all right. They've got
Black. He's locked away upstairs. The dementors will be performing the
kiss any moment now "
Second - memories can be modified well. Dumbledore notes that Slughorn - as capable a wizard as he is - performed the modification on himself ineptly.
"Because, I think, he is ashamed of what he remembers," said
Dumbledore. "He has tried to rework the memory to show himself in a
better light, obliterating those parts which he does not wish me to
see. It is, as you will have noticed, very crudely done, and that is
all to the good, for it shows that the true memory is still there
beneath the alterations.
Presumably some forgeries are (almost) impossible to detect, as evidenced by the fact that Tom convinces Morfin that he killed the Riddles.
"We have no memories to show us this,
but I think we can be fairly sure what happened. Voldemort Stupefied
his uncle, took his wand, and proceeded across the valley to 'the big
house over the way.' There he murdered the Muggle man who had
abandoned his witch mother, and, for good measure, his Muggle
grandparents, thus obliterating the last of the unworthy Riddle line
and revenging himself upon the father who never wanted him. Then he
returned to the Gaunt hovel, performed the complex bit of magic that
would implant a false memory in his uncle's mind, laid Morfin's wand
beside its unconscious owner, pocketed the ancient ring he wore, and
"And Morfin never realized he hadn't done it?
"Never," said Dumbledore. "He gave, as I say, a full and boastful