An Inter-Metagalactic League?

Why does the universe seem to be so underpopulated in James Blish's Cities In Flight? There is a handful of intelligent species in the Milky Way but no new races are encountered either during the colonisation of the Greater Magellanic Cloud or during a brief passage through the Andromeda galaxy and there is not enough time to explore any further - but why is there no mention of any Dirac messages received from other civilisations?

The only two species that travel to the metagalactic centre before the Ginnangu-Gap have originated in the Milky Way. Blish agreed that this was an unacceptable coincidence and suggested that it might have been dealt with when the books were filmed.

I would not have welcomed a film adaptation that simply added a Babel of extra species to the human beings and Herculeans. I envisage instead an inter-metagalactic league of spacefarers using spindizzies to move artificial structures larger than planets, monitoring human Dirac messages and maybe rescuing the Herculeans who would otherwise have been killed by the Hevians' counterattack. Some spacefarers would contemplate the Ginnangu-Gap with equanimity. Others might know how to escape to other universes if they wanted to?

But it is hard to imagine genuinely alien beings and civilisations and I think that Blish was right to exercise restraint on that issue in this series.