I have two takes on it:

1) We don't know the possible implication of new discoveries. Well, actually we get some glimpse of it, for example with nuclear or biological weapons around, it is fairly easy to wipe out a large part of the population. The more advanced the technology becomes it seems there is a higher chance of it blowing up in our faces and us destroying ourselves. So the galaxy could be colonizable in a few million years, but any civilization headed closer to that ability, gets closer (and faster) to its own destruction as well.

> The only logical conclusion is that there are no other technologically advanced species in the Milky Way or that we're the first(very unlikely).

Given my other pessimistic take on it (1), I would say it likely there were (are) quite a few technologically advanced species, but they've wiped themselves out or will do so before the reach the ability to colonize planets in other galaxies.

2) Another take on it is these civilization have to share the same physics and chemistry that we do. And maybe there is just a limit that gets achieved and long term colonization or fast travel is just no possible. That is there are no shortcuts like wormholes, Alcubierre drives, antigravity and other such things. We've been used to seeing huge leaps in technology and science in the last 1000 years, so it's tempting to extrapolate and but maybe we are close to reaching a wall. So there are quite a few civilizations out there but they've flown probes in their own start system, now developed internet and are sharing pictures of alien cats, hanging out on their version of Facebook and watching their alien Netflix, and of course discussing their equivalent of Fermi's Paradox on online forums. All without a chance of ever leaving their neighborhood.