... in the Letter Writng section of this site, if you 'hover' your cursor over the complimentary close, it shows as a capital! [e.g., Sincerely Yours]
Thank you for bringing that to our attention, Colin. I've notified our editor. Let's see what he can suggest.
As for the reason writers today capitalize both words in the "phrase" Yours Sincerely, it may have to do with consistency. The opening is always capitalized; e.g., Dear Sam; To Whom It May Concern; Hello Max; and so on. So by analogy it seems that writers carry that style over to the closing; e.g., Yours Sincerely.