1080 ti very close on both efficiency and hash/$ on MANY algorithms with anything else even as a bare card - don't ASSUME that all mining is Ethash (ETH/ETC/ECL and such) where the 1080 is a bad choice.
It also offers higher rig density than any other option that is affordable (Titan V is NOT affordable) for which some folks are willing to pay a small premium vs "max efficiency" cards.
Can also depend on what is AVAILABLE the last 3 months - sometimes you buy what you CAN get, not wait a month or two to sneak in a buy on something you would prefer IF you get lucky.
I'd rather wait a month or two instead of waiting for years for ROI. At no point have 1080ti's been anywhere close to being the most cost effective mining card, even with limited options, even considering electricity cost, it just doesn't work out.
A GTX 1080 ti is VERY cost effective on merged folding - probably the best card to use for that - *IF* you have a folding-optimized rig.
On ZEC, they're very close on hash/watt vs ANY member of the GTX 10xx family when all are operated at "most efficient" settings, and they've usually been very close on a "rig-level" basis on hash/$.
THE most cost effective, no - but not all that far off it and some folks are willing to pay a small premium for the higher rig density they offer (and lower time spent managing those rigs on LARGE farms).
The numbers probably vary if you're not in the US - some areas might charge more of a "premium" for the higher-end cards than we usually see here.