Actually, this isn't true. There are units that have had Stat changes.
The new Dark Eldar Archon will go from WS/BS:6 to WS/BS:7 (and may have higher attacks and/or leadership).
I think Minotaurs/Rat Ogres/Tyranid Warriors/Hive Tyrants all had slight stat changes (or at least special rules that effectively change their stats and/or play style).
When you have very old established stat-lines (like human or space marine), they are unlikely to change. When it comes to things like Necrons, people aren't going to be upset if they changed around (maybe even have a unique looking stat-line); there's no fundimental 'Cron' statistics.