Over time, the Trek producers painted themselves into a corner with their prehistory, one that mattered little when said history was just a matter of the occasional "Back in the days..." reference, but a great deal once they began dramatizing some of those events.
Again, it tracks back to "Balance of Terror," as a surprising amount of Trek continuity does.
In the aforementioned speech about the Romulans, Spock indicated that Vulcans had been in space for possibly thousands, certainly many hundreds of years - their "early colonizing period."
And then, the writers of
First Contact exacerbated that by making the Vulcans the first human encounter with extraterrestrial beings.
Which meant that when we first ventured into space it was not a mysterious, unexplored territory at all. We had guides from the beginning.
Which contradicted the essential Trek-ish premise of humankind venturing out into a frontier, exploring strange new worlds, seeking out new life, yada yada.
So...the creators of
Enterprise had to justify the fact that the Vulcans were of little help to us. And they attacked it from two angles:
- The Vulcans "aren't explorers" - they just lack the curiosity, or they're too cautious or whatever. There were many, many reachable places they hadn't been; and
- The Vulcans are dicks, and don't want to help us, don't want to provide information to us.