Because it was the shortest route. Space be big, man.
But that makes no sense. We saw how ships move through this "shortest route" - at a walking pace. To say that a detour a
thousand times longer would also be a
thousand times faster is the understatement of the aeon!
To spell it out, if the rubble field lay only between Yorktown and Altamid, and Kirk took a route around that was, say, a thousand lightyears longer, he'd be at Yorktown thousands of centuries before Krall at the speeds witnessed. Of course, since Krall did get to Yorktown in a matter of hours rather than geological ages,
part of the rubble field must be easier going - but that doesn't alter the fact that other parts, explicitly and visually near the respective endpoints, do slow starships to a crawl, and the fantastically faster warp speeds then make any and all "long routes" massively more attractive.
And there is no evidence either visual, nor through dialog that puts Altamid in the 'Nebula'.
Except for the fact that everybody feels they have to go through the nebula to get there. We never see any evidence of a clear route, eiher visual or dialogue - and there would be an obvious plot requirement for
using such a clear route yet at least three separate parties fail to make use or mention of it.
Oh, and I re-watched the movie last night and the asteroid field runs through the entirety of the 'Nebula' and is not limited to it's outer edges.
The movie only shows us snippets of the nebula, never a single solid run through it. All we know is what it looks like near Altamid (the first time our heroes are shown dodging the rocks) and near Yorktown (the first time Krall is shown emerging from the rubble) plus random bits from the time McCoy/Spock and the
Franklin brave the field. And that already is solid proof that the entirety of the nebula
can't be like that - because Altamid and Yorktown are in different star systems visually, and "rubble speeds" would make the transit between them so long that both stars would die out in the meantime.
As a thought experiment, what would it take to make the nebula the fastest route? Let's assume that the long and clear way around could only be traversed at one-third lightspeed for whatever reason. Let's also say the nebula is a wide flat disk that is, say, a thousand times wider than it is thick. So it's a thousand units around, one unit through. The speed through could be generously estimated as 100 m/s, vs. the 100,000,000 m/s on the way around. See the problem? The nebula would have to be more like a million times wider than it is thick for the math to have any chance of working. And that's before evoking warp, which should be available for at least part of the way even if we postulate that Yorktown and Altamid orbit the same star.
If Starfleet was interested in what lay behind the rubble curtain, why not establish Yorktown at its edge instead of the middle? A warp speed peek around would then be trivially easy and quick.
Timo Saloniemi