One gets the impression they'd rather not accept Railcards at all if they had the option.
That's basically how it already is
if you're not travelling alone...
I can never seem to find any Railcard discounted tickets if I search for one berth with two 16-25 Railcard holders, for example. Very frustrating if I wanted to do a weekend away with someone!
I don't quite understand the reasoning behind it, considering it just means you could instead book two individual cabins for just £10 more most of the time, and that means you're just taking away a decent amount of revenue from the sleeper which is already rather financially unviable.
See this example...
£310 for 2 adults/1 room with no Railcard
or 2x 16-25 Railcards in Classic
£319.60 for 2 adults/2 rooms both with 16-25 Railcards in Classic
Thankfully I have other discounts available to me, but I can certainly understand why this would put people off.