Red alert 2 was released in 2000, which means it was developed probably in 1998->1999.

Life was different back then. Source control systems were a LOT worse (this is CVS era for open source development). We certainly weren't in the current era where everyone wants to play these old games on their phones now.

What almost certainly happened is all copies of the source were simply lost. They may have sat on someone's hard drive, a team server hard drive, or somewhere else. It's possible they didn't have more than a few copies while development was ongoing.

Everything will be lost unless it's somebody's job to preserve it. It's pretty common to close game studios or lay off the entire development team while simultaneously scolding them about "stealing" IP so the predictable result is the code being lost.