Parsees in India is decreasing

For stable population growth, a fertility rate of 2.1 is required. That means, every couple should have 2 children. So, 2 people will give birth to two children, which will keep the population stable. 

The fertility rate for Parsi community is 0.8, much less than the required 2. That means, 2 people from Parsi community will produce 1 child, which is a natural reason of decrease. For every 9 Parsi families, there in only 1 child below 10years of age.

Late marriage may be the reason as well. Minimum age at which Parsis get married is 27 for girls and 32 for boys. This is another reason for low fertility rates.

According to a legend, when parsis arrived in India, they promised the local king that they will not try to propogate their religion. Parsis in India are so exclusive as a community, that until recently, they did not accept marriages of Parsis with non-Parsis. Their children were not regarded as Parsis, even if the Father was a Parsi. Due to these reasons, relative marriages - Uncle with neice or between close cousins became common. And as it is biologically not viable to produce children by two people sharing a common genepool, naturally, their numbers dwindled.

GOI started an Initiative, JIYO PARSI, where the Govt encourages early marriage, having children early, having a second as well as a third child, as also medical support upto 5 lakhs against difficulty in child bearing.