Why mara wants to distract These davas are always against Buddha, his Dhamma and those who are practicing.
Since they also exist in planes of existence, why do they bother others trying to liberate themselves? They themselves might become a monk in next life.


  These devas enjoy sense pleasures created by others for them. Mara,
  the personification of delusion and desire, lives here.
I think your question itself has an answer. 


  These devas enjoy sense pleasures created by others for them


The closer a being gets to Nibbana, the less sense pleasure he/she will create for Mara. That's probably why he doesn't like beings attaining Nibbana. But he seems to encourage beings to do general good deeds and aspire for heavenly existences.

The Paranimmita Wasawarti realm where the Mara is at has a lifespan of 9216000000 human years. So his next life is not going to come for a long time. But when it does, it'll most probably be one of the hells. Not human realm where one can be a monk. He'll probably be around to trouble Buddha Maitreya as well.