Question

why do spores remains viable during unfavorable conditions ?

Answer 1

Due to the presence of sporopollenin in pollen grains . No acid, base, enzymes can degrade it . It is a polymer of carotenoids.

Answer 2

The spores remain viable during the unfavourable condition because of the presence of the sporopollenin in the pollen grains.

It is the polymer of carotenoids.  

When it comes Sporopollenin, it is a major component of the exine of pollen and spores.

It is highly resistant component available in this organic world.  

Also, it is resistant to the non-oxidative enzymes, chemicals and other chemical reagents.