In threads here, on Reddit and in comments on news sites, a lot of new information is measured by one big misconception. Well, unfounded assumption at least. A lot of people seem to be convinced that the colonists leave for Andromeda because of the Reaper thread.
Of course this isn't a crazy idea, but it's still very much unconfirmed (it's not even mentioned in one of the many leaks). I have the feeling this isn't the primary reason the Andromeda Initiative is initiated. There could be other reasons. Exploration for the sake of exploration. The fear that dark energy would make the Milky Way unhabbitable. Fear for the Geth after the events of ME1. There are alternatives.
All we know is that Ryder leaves before big choices are made in Mass Effect 3.
It's too premature to get outraged about: 'But no one believed Shepard about the Reapers!' 'Why would've no one told Shepard!' 'Why wouldn't they put all their energy into the Crucible project!'
Who knows, perhaps the colonists aren't running in panick from Reaper thread and are just like 'yea, ok, I'll come along' and have all the time in the world to prepare.
I think you have a point in that we don't know for sure, but what other reasons could there be?
Exploration? Not really feasible since communications with MW is non-existent, so essentially no exploration info would make it back to the MW. All the MW races would be doing is sinking an incredibly unimaginable amount of money into a project that would not only have 0 return on their investment, but they would never even know if the Arks arrived safely.
Dark Matter? Could be, and I'd like it if it were related to this, but I think BioWare decided to drop this whole plot point when they changed ME3's Reapers to be stupid standard evil robots with nothing interesting happening.
Right now, leaving because of the Reapers is pretty much the only theory that makes sense, so for the moment people assume it is true. There isn't anything wrong with this because we know next to nothing about the game anyway.
Fear of the Geth? Also no, because the Geth were beaten by the Council between ME1 and ME2, they were forced to retreat and everyone else is now confident they could take them.
'But no one believed Shepard about the Reapers!' 'Why would've no one told Shepard!' 'Why wouldn't they put all their energy into the Crucible project!'
We meet people who did believe Shep, they just keep it on the dl because they don't want to sound like crazy loons.
Shepard was in jail/house-arrest/under-suspicion-of-being-cerberus at the time so it makes 100% sense that no-one would tell him`.
Sometimes you can't put more effort into something. As a Software Engineer it is blatantly obvious that sometimes adding more people to a team would only end up slowing it down. Plus the whole don't put your eggs in one basket, especially when that basket is powered by space magic you don't understand, and everyone is really just hoping it works because no-one has any kind of clue what exactly they are making.
Not saying it is 100% 'run from Reapers!' I am just saying it is the best theory we've got measuring other info by it makes sense. And berating people for it makes no sense.