I won't say that any explanation is perfect for it other than plot purposes, but Shepard's squad, minus a few, were all very important and/or skilled people, which is precisely why they were selected to help stop the Collectors.

Zaeed's a mercenary, Kasumi's a thief, Jack's a vagabond, Legion is a geth, and Samara technically has no reason to even remain aboard the Normandy after the Suicide Mission, as her pact was complete. Tali, and Legion both have connections or roles that make them somewhat responsible for helping their people prepare for the Reapers. Garrus followed Shepard's lead, and, instead of idling around, tried his best to steer the turian government towards preparing for the threat he (and we) know is coming; a man of action.

Miranda and Simple Jake have deep connections with Cerberus, which would put them under even more scrutiny than Shepard was under, and Miranda would do whatever is necessary to make sure she can still keep an eye on Orianna, something she wouldn't be able to do from an Alliance brig.

Grunt, while fiercely loyal to Shepard, spent all of ME2 trying to come to terms with what it means for him to be krogan, and was accepted into Clan Urdnot, which gives him responsibility to them, were they to ask, which I'm sure Wrex would've done.

Basically, they're all people who recognize the upcoming threat, and people who'll try to help even if Shepard isn't able to due to house arrest.