There are a variety of reasons I assume. One is due to numerous negative stereotypes associated with black people, but surely one cannot believe a stereotype is representative of all or even a majority of black people.
Another reason is due to a culture clash, but being in an interracial relationship doesn't mean you have to personally lose your cultural roots. I am black British with Caribbean roots and I have been with an Indian girl for 2 years (her parents don't know). Should we procreate in the future we will raise our children with knowledge of both of their cultural roots and teach them about both my beliefs as a Christian and her beliefs as a Hindu. When they grow up they can choose which religion they wish to be (if any religion at all.) What matters more is that my children are genuinely good people who can be proud to be British and proud of both their roots.
Lastly, I would say pure ignorance on their part is an almighty reason. The unwillingness to accept that their are genuinely good hard working black people out their who would make admirable and quality partners for their children despite any culture clashes. The fact some are so stuck in their ways that they are blind to the actual character of a person and can't see past race.
Btw, although a significant proportion are like this, please be aware not all South Asian parents hold these views . My cousin is happily married to an Indian girl and they have 2 beautiful children and he never had any trouble from her parents.