Why was the entry level line at Audio Research eliminated?

Prima Luna.
I'd say so. 
Ck out the bidding on PL units,, Hot items. That is one brand which retains most,,well lets say a higher resale margin vs many other brands. PL is made in Spark lab,,, where Cayin is made, which i can testify matches up to any famous high priced american labs. 
I'd put my Cayin CD17 Mark 1(modded with new caps and new Descrete opamps. = $700 mod) ) up to any Cary cdp. in a shootout. 

If we're mentioning favorite brands in this category, I'll go with Quicksilver.

Audio Research has always been about Reference level products with some exceptions. In the past 15 years they haven’t produced much below that reference threshold they have set for themselves. I think most of it has to do with what they believe the minimum level they can achieve and still be considered reference by their standards.

Don’t forget Schiit has tube preamps...

I'm not sure about the entry level models specifically, but ARC have removed sku's for the REF10 phono and preamps and others I forget. A dealer I spoke with earlier this week said ARC have only 7-8 sku's now because parts have become unavailable.