its a combination of a lot of different things..
technology advancements in manufacturing...
technology advancements in the materials used..
technology advancements in ammunition (both manufacturing and in the materials used)
rise in demand for aftermarket products that are both affordable and improve accuracy (everything from triggers to stocks)..
technology advancements in manufacturing that allow lower cost optics to actually be better in performance than many "high dollar" optics were even just 20 years ago..
put all that together.. and you have a more consistent and accurate rife, that can easily and affordably be "ugraded" to an even higher level of performance, that shoots ammunition that is more consistent and accurate, that is sighted onto its targets by clearer, more reliable, and more consistent optics (whether we are talking about peeps, red dots, or adjustable scopes..)..
all at a relatively low price..
honestly, with the exception of a couple of true precision, custom rifles I had built back in the late 90's by a true master gunsmith.. as a general rule, any of my "budget" rifles (mossberg, handi's, etc..) can shoot just as well as my completely tricked out "high dollar" bolt guns..
as a result, I often choose to take one of my $350 rifles to the woods to get it wet, knocked around, and beat up on a hunt as opposed to taking one of my $1000-$2000 cool guy guns.. for the most part my high end guns have become safe queens these days and only get taken out occasionally.. my "cheap" rifles get shot all the time though..