Why does Peter’s quote of Deuteronomy provide evidence that he accepts Deuteronomy as scripture, but Jude’s quote of Enoch doesn’t provide evidence he accepts Enoch as scripture?
Peter quoting Deuteronomy is one reason we can say Peter thought Deuteronomy is Scripture. But taken alone it doesn't necessarily mean he believes it is Scripture. Another reason we can say the Peter is quoting "Scripture" is that we know he was a Palestinian Jew and we know, historically, that all Jews believed that Deuteronomy is scripture.
We also know Deuteronomy is Scripture because Jesus quoted Deuteronomy often.
So how do we know Deuteronomy is Scripture.
Jesus Quoted it as "It is Written"
Peter Quoted it
We have historical confirmation that it was accepted by the Jews as Scripture
It was universally accepted by Christians as Scripture
Even the Old Testament books that aren't quoted in the New Testament we know that:
Historically, they were accepted by the Jews as Scripture
A consensus developed in the Christian church (except for the DC books) that they are Scripture
With Enoch we know that
Jude quotes it
It was widely read by the Jews
What we don't have any proof of is that Jude or the Jews considered is Scripture and it certainly was never universally accepted by Christians as Scripture. It could be that the people he was talking about in his letter accepted Enoch as scripture and Jude was using their own scripture against them to show they have rejected God.
It could be (and probably is) Jude make a cultural/historical reference instead of a scriptural reference.
Just because something is quote or alluded to by the New Testament writers does not make it Scripture.
The Jews had stories and legends and historical writings and books just like we do today. And just like contemporary writers use those things to make a point or give context to writing/speech/sermon the writers of the New Testament, even Jesus himself, did the same thing.
If a respected preacher quotes "The Lion, the Witch, and the Wardrobe" in his sermon and uses it to make a point about Christian ethics or Christian faith that doesn't mean the preacher considers it Scripture. Even though it is a great, even inspired, Christian allegory.