Preferring marriage within the same cast has to do more with having partner from the same cultural/family norms and practices. You see, Indian society is quite old (thousands of years); there are lot of customs re ways of worship, deities, dresses, eating/etc. that have evolved over this period and people (mostly parents) prefer a match in that regard. Because mostly marriages are arranged by parents (though in cities at least, love marriage has also become a common place thing.)