Theoretical: The space between galaxies is expanding according to Hubble's Law. But more than that, the expansion rate is accelerating. Hence the distance that you think you need to traverse to get to another galaxy now will be even larger in the future.
Hypothesis: Codex: Tyranids suggests that the Great Devourer came from the voids between galaxies. i.e. they have traversed this great distance. Whilst not impossible in a physical sense (i.e. they'd take less than the age of the Universe to do so), it is probable that they came from only the nearest galaxies if this is true. Hence it may be physically difficult to accept that the tyranids are attacking the Milky Way from multiple angles (out of the plane of the Milky Way's disk, and along the plane) as that would suggest multiple tyranid origins (i.e. they're come at us from different galaxies on opposite sides of the Milky Way). So, I'd like to propose a new hypothesis: tyranids come from Galactic Cannibalism. We know that the Milky Way is gobbling up the Large and Small Magellanic clouds as well as the Sagittarius Dwarf galaxy. Hence I'd like to propose that the tyranids originated in one of those gobbled up galaxies. This solves the issue of the tyranids coming at us from multiple directions with grace -- the destruction of their original galaxies would sling shot those original stars throughout the Milky Way with ease. And Occam's razor suggests it is the better (simpler!) explanation :)