Tidal - why only remastered versions?

I think it may very well depend.

Usually I find both. For instance, I just searched for "Little Earthquakes" by Tori Amos and found both.

This is because Tidal gets the music from the record labels. The only version of this music now sold through retail and online outlets are the remastered copies, as they are the labels excuse for selling you the same recorded music again. It's only natural the label would want to provide Tidal with the latest remaster, as many people these days will use streaming services to determine which music they actually want to own, and are more likely to buy whatever version they played on the streaming service. 

Hey Mark
  I don’t mean to derail your thread but I am curious as to why you prefer original masterings.  I primarily listen to Classical and usually the remasters are far superior to the originals (which in the fifties through mid seventies were primarily mastered to be played on AM Radio), but pop is a different kettle of fish.  

yea some remasters are pretty awesome... echo and the bunnymen are great
megadeth, no... but there is a story on those
the cars remasters are great