Christianity in India is often framed against the prevailing religion Hinduism, for better or worse. The Indian church emphasizes things like social equality, income equality, etc. Some of the earliest labor activists and trade unionists in India were Catholic (actually Catholics in general are generally pro union across the world, see the Catholic vote here in America).
Secondly, Catholics are often setting up schools for everyone. India has always had a history of education, especially Kerala, but universal education of even the lower classes is extremely protestant. The church ended up adopting this around the time colonialism started and thus brought universal education to a widespread base in India.
Finally, the idea of touching everyone and treating them equally was against the general zeitgeist of the prevailing feudalistic highly hierarchical indian society. The first conversion attempts of the Portuguese for the south indian brahmins actually were incredibly successful (Nicholas of Tolentino). The Vatican even allows (and still allows as far as I'm aware, although no one does it) vedic rites for Catholics (malabar rites controversy).
However, no one wanted to give up untouchability. The Vatican eventually forced the missionaries to not have separate missions for touchables and untouchables, which basically ended Brahmanic conversions (and is one of the reasons indian Catholics no longer really care to do the vedic rites, since most are now from the lower class. As far as I know, some still do in Mangalore). Caste is still a problem in some christian communities in India but the bishops work to end it and it is officially condemned.
Which is to say, catholicism is associated with labor movement, equal social treatment, and universal education.
Which is also what the communists want.
It's no surprise that Kerala, being way more christian (and Catholic particularly) with a rich and prominent Christian history is thus the center of socialism.
Keep in mind also that communism in feudal countries has basically no relation to the communism you find on university campuses of america.
Now to the west. In the west, the church is seen as conservative, but the church is actually radically left wing in most parts of the world. It's only because leftism (in a global sense) is fundamentally a part of western culture that the church seems right wing because the church does not go as far as some leftist parties in the west.