The WAU presumably revived him as a "new human" rather than a dead staff member, perhaps to see if he retains his sanity where dead crew tend to go mad as they know they're dead? Yeah this doesn't make an awful lot of sense to me. Or maybe it just selects a scan at random, we'll never know for sure and for things like this all we can do is speculate I guess the WAU wanted test the new design with a scan that was more likely to be able to handle it all then risk another insane robot wandering around. The WAU realised that simply uploading scans into the helper robots wasn't going to work so it set about developing a more human-like body so it wouldn't be too wierd and hopefully prevent whoever was uplaoded from going insane (hence the diving suit + dead body). Consequently, when uploaded into a robot they would just go insane. The method that the WAU had been using to scan the members of PATHOS was its own much more improved version of the brain scan which captured a much more complex scan. Cath describes simons (and her own) scan at one point in the game as "flatter and less emotionally complex" which is why he is able to handle knowing he is a robot without going insane like the other robots you find.
Simons brain scan is a 'legacy' scan meaning it was one of the first and earliest brain scans that was used as a template for developing AI, which is why the WAU had a copy of it in the first place.