Nemestice is a temporary mode that came to Dota players during this years battle pass. A large portion of the Dota community has fallen in love with this mode and some have chosen to exclusively play Nemestice. We think that Nemestice could easily replace a mode like All Random Deathmatch in the unranked section.
Here is a brief description of the Nemestice mode:

GATHER METEORITE FRAGMENTS
Every three minutes, the Nemestice Storm will cause a giant meteorite strike in the center of the map. Channel meteorites to collect Nemestice Embers. Try to dodge any falling meteorites before they hit you.

POWER FROM THE STORM
Nemestice Embers grant Embercharge, boosting your attack damage, spell amplification, and movement speed. Gain enough Embercharge and you'll even shock nearby opponents.

NEMESTICE TOWERS AND CREEPS
Each time a tower is destroyed, power disperses to its allied towers, making them stronger—as well as drawing the attention and assistance of Nemestice-crazed creeps. Destroy all enemy towers to claim the area and win.

Here are some reasons why people prefer Nemestice.
Faster game times than normal modes: 
       Normal modes(45min - 75 min)
       Nemestice(8min - 25min
Faster Queue times
More consistent game pace/progression
Partially shared Team Gold/Exp
Fun constant Teamplay
Ability to try a variety of builds legitimately
Fastpaced Strategy between laning and team objectives(Crystals)
Less PVE more PVP

Nemestice is in a lot of players opinions just more fun and rewarding and want to see it become a staple of Dota 2 permanently.