Shifting direction is different for front and rear shifters
The reason why shifting is arranged this way:

Shifting to a larger chainring at the front gives a higher gear ratio, shifting to a larger sprocket at the rear gives a lower gear ratio.

Shifting to larger chainring/sprocket requires positive pulling force from the shifter whereas shifting to smaller chainring/sprocket can be done with derailleur spring tension. It's intuitive that getting the chain to  climb up onto a larger chainring/sprocket will takes more force than letting it fall down.

Its easier to get positive pulling force from the thumb shifter lever then the finger lever, there's more room for a longer lever and a longer lever throw. The thumb can travel on longer distance without the  hand losing grip.


The exception is of course Shimano's Rapid Rise rear derailleur which use derailleur spring tension to shift to larger sprockets. THis obviously didn't work as well as 'normal' derailleurs as the type has disappeared.