I think it's not so much the singer, but the songs ..
If you look at her album list, it's got a pretty good mix of easy-listening pop tunes that sound catchy, but haven't been enjoying airplay lately. So, there's that 'nostalgic' appeal.. And of course, add to that the 'bossa' touch ..
So, there's that feeling of: 'hey, I know that song, but it sounds different! Who is it?'
Not sure if it's a good comparison, but it could probably the same formula that propelled Nina's 'Live' album to the top of the charts not too long ago.
Bossa nova has definitely been around for a long time. So many unknowns who can probably do it ten times better have all but vanished into oblivion..