Oh, I get that part. What I don't see is why attacking them first was wise.
Think about it, at just the risk of attack all the other races pretty much abandoned Earth, and that's before they knew that the bulk of the fleet was there. And the galactic politics were already beginning to flounder. But Humans were more-or-less the token Citadel race--we didn't have the same levels of technology, resources, colonization, economy, etc. as the rest of the leading species.
Why not target Thessia first? That utterly destroys the galactic economy, removes the galaxy's best mediators from the equation, and severely limits the ability for the resistance by destroying a significant part of their infrastructure. Though, to be fair, perhaps it's because the Asari had colonized so many worlds, where humans had colonized few. But I doubt that would have been such a problem if the goal was to stop unification, not to outright destroy the Asari.
And Sur'kesh would have been an even easier target. Salarians were amazing when dealing with your typical mortal, but against a behemoth like a reaper just about everything they could do was rendered completely obsolete. Subterfuge means little against them. And because of their particular biological needs, Salarian colonization wasn't as extensive as Asari. And targetting them would be a major blow to the staging of supply lines throughout the galaxy.
Instead they sent the bulk of their fleet to Earth, with most of the rest to Palaven. Palaven could hold its own well enough, given the fact that nearly every Turian was combat trained, their population was huge, their fleets vast and powerful. Not to mention they were decently well colonized, at least in their sector. The fleet they sent to the planet didn't even put their species out of commission with regards to galactic politics and infrastructure--seems to me that same pressure could have done WAY more damage if directed at either of the other species.
And then there is humanity. Less advanced technologically, smaller/weaker fleets (the Normandy, afterall, was a technological marvel to all races, but far closer to Turian designs than Human), fewer dreadnoughts (iirc), much more limited colonization than the other races due to time constraints. While they wanted to harvest humans, they also posed the least risk in that cycle.
Why not take out the Salarians to severely weaken the three remaining species, then divide most of your troops between Asari and Turian sectors, always maintaining at least token forces in the Sol system? That way, the Alliance is still too busy protecting itself to actively seek help, and the rest of the Citadel forces are way too occupied fortifying their borders or saving themselves to even seriously consider helping? If the decimation of a minor race was a motivator to stay out of it, imagine what the decimation of a leading species would mean? They could still have started collection, just at a slower pace. But reaper presence still means indoctrination, and indoctrination means regimes will fall from within regardless of outside interference, given enough time. Send enough ships to move from city to city, so they are always defending or recuperating, and focus on Alliance hubs like Vancouver. That way, they can never really muster the ability to seek aid.
That's my take on it at least. But in my opinion, when you can so easily destroy infrastructure because major hubs are SO exposed like Thessia and Sur'kesh were, at least when compared to (say) Palaven, why wouldn't you? Earth was always the smallest risk. MAYBE more than Sur'kesh was, but with a fraction of the ability to organize the non-Council species.
And it's not like Earth could have really done anything to protect their colonies in any case--those are easy human targets right there...