Without knowing what episodes the screenshots came from, it’s hard to say - but it seems to me that the more detailed backgrounds tend to be used for establishing shots, like that gorgeous shot of Fluttershy’s cottage at dusk we got last week. If characters need to interact with parts of the scenery, the scenery will usually be simpler, because that makes it easier to animate.

The season 2 shot is taking place during the fall. It still looks like it’s the exact same art asset, but it’s been recolored for different lighting conditions, and to reflect the Zap Apple harvesting season.

@Background Pony #7BDB  
then the second is true.
 
To save money.

@IpersonallylikeCupcakes  
The did use it, in family appreciation day, they used the telescope.

The tree looks better in the middle shot too.

I think it’s because they realized they never used that second set of stairs.
 
Either that or to save money.

cuz its apple season

Probably pressed for time.