I'm moving a ton of people (asteroid miners and affiliated personnel living in the Asteroid Belt) onto a generation ship which will take years to reach its cruising speed of .1c, and which is part of a fleet numbering hundreds of ships that's prepared for a three thousand year journey or more. My story takes place partway through the second millennium of the journey, and despite over twelve hundred years having passed since departure, none of the fleet's citizens have become posthuman like the entities which built the ships in the twenty-sixth to twenty-eighth centuries. Furthermore, science and technology haven't even significantly advanced by more than a relative century's worth of development since departure.
How might I explain this complete and utter stagnation? Is there any conceivable reason the entire fleet hasn't budged an inch technologically since they left the Solar System over a thousand years ago?
Additional information from comments by the OP:
The fleet stays in regular contact with each other though, relaying occasional brief status updates a few times every Earth year. Usually they include any new, relevant or important information they think the rest of the fleet should know. Each ship has a population between 1 and 10 million people as well, meaning the total sum of the fleet's population is about a billion.
Each ship is about the size of a major city. Even the small ones carry hundreds of thousands. And this is after a thousand years of population growth, mind you. The original crew of each vessel probably didn't exceed 40k back when they first launched.