Why was the entry level line at Audio Research eliminated?

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. 

Probably more to do with the new owners, and more focus on a world market than an American owned company catering to US sales. Not saying the direction they have taken wasn’t a wise one, but gone are the days calling them to ask a question, send in an old discontinued piece to get repaired or order a part directly. Even my local dealer has a heck of a time getting access to the new company. 
For the money a company like ModWright builds as good as or better than ARC, and there’s others as mentioned. ModWright is a great example that equipment can be made in The States that is built well, sounds great and at a reasonable price.