Seems the only points of "biblical" citation for the Gospels are the OT...and those are a rambling set of allusions, indirect quotations, and ranging paraphrases. (Which probably has more to do with oral tradition and lack of scrolls with the biblical texts. Also, who has time to unfurl Isaiah's scroll to quote from his last chapter?)
The Pastorals are notoriously difficult to date (I accept Pauline authorship) but they are clearly among the last. Paul seems more open to quoting sources other than the OT. The Gospel writers seem only to pull from the OT. How that works would be a great dissertation topic.
Maybe that is it though, and it reinforces my first point, the nature of epistolary literature vs. ancient biography and their sources for authority. Wow, makes me want to apply to a European school for a second PhD...and apparently a new marriage since my wife has refused such requests on grounds of her sanity.
The genric difference are likely the key. The epistles of the NT quote from within and without. The Gospels, and Acts, don't do this. Then we get to Revelation and all rules and thoughts go out the window.
It would be interesting to see what Beale has to say about all of this.