My answer is around 2010 that Colt started to loose their luster. They had always been the standard but they were sold at a premium, sometimes double the price of other manufacturers.
A lot of AR manufacturers have risen and fallen over the years...DPMS, Olympic, and Bushmaster were hot in the 90's, then along came Rock River and Stag. The Chart was put together and published around that time showing the little corners that some of those companies cut when compared to Colt. At that time a Colt 6920 was around $1,300.00 but they were considered the cream of the carbine crop until...
BCM, LMT, and Daniel Defense showed up on the scene with carbines that were just as good as Colt, met all the criteria on The Chart, but with some improved features. Some, like me, want a midlength gas system and a free floated handguard, etc. Big brands like Sig, Ruger, and S&W jumped in with models that offered these features at a more affordable price and better yet, were just as reliable.
Along came 2008 and there was was an election scare- Obama got elected. More than ever, lower receivers became a hot commodity. The internet spread the word that a customer could buy a lower or ten and stick them in the safe for a later build.
Youtube showed how easy it was to buy a bunch of parts and build your own. Palmetto State, Aero, and Anderson came on the scene offering parts and kits at rock bottom prices. If it didn't run, no problem, there is a huge community of online gun builders that could assist you. Now the hotness is the AR pistol, something that Colt has not even attempted to get in on.
Colt still makes a solid product, and I own one, but it is not my first choice for some of the reasons I mentioned.