To answer the OP's post …
PF on the subject of the Caldari and Achurans states that SuVee settled on Saisio shortly after the evacuation of Caldari Prime
[1]. The Achura/SuVee relationship has thus existed for a little short of two centuries, but in all likelihood it may have existed well before that. Additionally, the EVElopedia entry for the Achura says they've been a member of the Caldari State for another century before that
[2].
That's a lot of time to build good relations on all sides, and I think that shows in what we see of their present-day relationship. SuVee traded higher-ranking corporate positions (not to mention adopting some elements of Achuran culture), in exchange for the right to call Saisio their HQ. They brought with them their economic influence and quite probably a lot of employment opportunities in their subsidiary corporations. Remember that the megacorps provide for their own workers (although whether it's truly beneficial in the end is up for debate along the typical Caldari v. Gallente lines).
In exchange the Achurans brought their scientific tendencies to the table. They may have also been willing to allow SuVee to settle (and remain over the years) because of one other thing: the megacorporations have this tendency to dig in and ruthlessly defend their interests. The Ingalles Incident was an excellent example of this
[3]. Each megacorp operates its own little fiefdom and doesn’t allow people to "rock the boat" within that. By remaining a part of the Caldari State, and allowing SuVee to retain Saisio as their headquarters, the Achurans (described as an introverted and reclusive society) are reasonably protected against the rest of the galaxy by SuVee.
You wouldn't find that sort of shield within the Gallente Federation.
The other thing I see as influencing the relationship is what they see "across the border" in the Federation. Under most circumstances the Gallente are
normally middle-of-the-road centrists, but PF shows that they have a nasty habit of extreme political swings in times of crisis. The rise of the ultra-nationalist government in the early days of the Caldari War of Secession was one example
[4]. More recently, I think we saw the rise of another ultra-nationalist government during the Foiritan administration and likely continuing into the current administration.
Ultra-nationalist governments do have a tendency to rock the boat in ways that disturb civil society, especially under times of extreme crisis. Sometimes a government like this, in my opinion, is good but sometimes it's not-so-good. A good example might be (in some people's interpretation, myself included) the present-day Russian Federation. A decade of economic/military/political decline hit the country hard, and it
especially hit the average man on the street badly. By the time that Vladimir Putin came along Russia was having a serious identity crisis. But the fact that he greatly overhauled the country (and is continuing to do so, albeit behind the scenes) means that the people will likely continue to remain permissive of the ultra-nationalist government that's leading them.
A "bad" example of an ultra-nationalist government, in my opinion, is what we see playing out in PF for the Gallente Federation. That being said, I understand the reasons
why the government came to be. The Gallente in the course of a year first were mauled by the Caldari. Next they were invaded by Uriam Kador's fleet, and then came Sansha's Nation. Their enemies are, literally, all around them to the point where civil society is probably perceived as being under threat. In response the political sphere within the Federation did a hard-right turn, which culminated in the formation of the Black Eagles
[5]. Remember that this is coming from a society which has in the past enjoyed an unparalleled and vibrant democracy. The Federation has never had the same sort of "citizen-warrior" mindset that the Caldari have, hence another reason why the people are probably looking for the sort of strong leadership that a temporary ultra-nationalist government might provide.
Of course, if PF is any judge to go off of, the ultra-nationalist government will
probably go away once these crisis have passed. Only to return the next time there's a series of crisis.
So, bottom line, I think the Achurans probably stayed with the Caldari State because of (1) the fact that siding with a megacorporation would protect and insulate their introverted society against the greater galaxy, (2) the economic benefits of having a megacorp using their world as its headquarters, and (3) the political volatility of the Gallente Federation.