If you worked in the defense industry, and dealt with sensitive information often, you would easily see how/why there is no data on an advanced project like the Eagle.
When the project is new and just warming up, it is considered 'Competition Sensitive.' The manufacturer has invested a ton of research time (spelled $$$$$$$$$$$$ ) and is hoping to win a contract and make a little profit. The last thing they want to do is publish a full set of data in the open press and give away all their homework and trade secrets to their competitors.
When the project wins the attention of a customer, it is generally something considered 'advanced' and is therefore highly classified. The Dept of Defense classifications severely limit any further distribution of the subject material, and carry serious penalties for anyone releasing the data to Popular Science or Fine Scale Modeler. Our government does not want our potential enemies to build the project before we do.
When the project is in early production, it can still be classified, plus everyone at the company has their hair on fire (getting the design finalized, fixing the production glitches, and getting the first articles delivered on schedule, all while simultaneously working on advanced versions to keep the production line open into the future). You want me to stop in the middle of this and provide documentation about WHAT??!!??
When production is rolling, and the customer is happy with their new toys, the security classification continues. It just won't do for our potential enemies to learn too much, too soon about our new widget, and develop countermeasures. Oh, by the way, since production is going smoothly and the stockholders expect to see profits, the company is going to have to trim costs by cutting back on staff. This includes most of the people who might be holding any project documentation.
When production is finished but the customer is still keeping the widget in service, the security classification may be slightly downgraded but will usually still be in place at some level. Sorry, no blueprints for "little Joey from Toledo," instead how about a copy of this (worthless) sales brochure that we handed out last year at the Paris Air Show?
Somewhere in this timeline the manufacturer is bought by another company, resulting in further personnel 'changes' and loss of data during housecleaning.
When the widget finally goes out of service and is completely declassified, there is no one left who was around when the widget was being designed. The company that did the work no longer exists. The government project office that managed the procurement and operation of the widget is closed. Imagine a voice echoing through an empty building: "Does anyone here remember the Widget?"
If you stop to think about it, it really is a lucky combination of freak events if any technical description of an advanced aerospace project makes it through the wall to the outside world.