Just my opinion,
1. It could be due to liquidity factor, Nifty and Bank Nifty are heavy hitters and have huge volume compared to others.
2. Comfort zone/Temperament
eople are just happy trading with one or two securities and they don't exactly feel the need to look at others.
3. Individual Nuances: Each security will have its own set of peculiarities or nuances and focusing on one or two will enable to understand it better.
Dumb Analogy for this would be: Sticking with your wife vs dumping her and looking at younger women. A known devil is better than an unknown angel.
4. Index Factor: Individual stocks will have many factors showing it's impact vis a vis the voltality. Example: Earning report could swing the stock value in 3+% percentages.
Compared to this the index is comparatively mellow as it is based on multiple sectors.
5. Manipulation: People feel individual stocks could be manipulated easily as compared to Indexes. Example: a board member knowing any new product introductions or earnings projections.
These are some of the reasons i could come up, some of thing may be wrong
... others might chip in...