What happened to hardtails in the past 15 years?
To paraphrase the question: Why aren't there any quality hard tail mountain bikes for $300?

Wouldn't it be reasonable to expect prices to go up since 2009?
None of the following examples are an endorsement. Only an attempt to point out that there are still less expensive good bikes available.

In an aluminum frame Trek has the Marlin 5 that sells for a little over $500.
It does have 29 inch tires but it does have triple chainrings and a seven speed cassette. 

If steel is OK then the Trek 820 is $400.
It has 26 inch wheels and triple chainrings with a seven speed cassette.

Maybe the shop you visited didn't have these bikes on the floor.
Less expensive name brand hard tails with triple chainrings do exist.

Most major brand bicycle companies define a type of bike and a price range they want to compete in. For example "road bikes in the $500 price range" would be a product slot in their lineup. In this case "mountain bikes in the $400 to $500 price range" would be a product slot. Specialized slots their less expensive mountain bikes between $600 and $750. All that to say - Trek isn't the only one with bikes in this slot.

The single chainring bikes you are seeing are the result of the increase in the number of speeds on cassettes and/or a focus on cross country or down hill riding. With as many as 12 speeds on the cassette some people have seen an opportunity to get rid of the front derailleur and simplify the bike for some types of riding.

Edit:
The question shifted while I was writing my answer!
I think this answer is still relevant.