There are three reasons why India and others might not be included:
1. It's in the name - CANZUK. There is no real movement for India or others to be included at all, so it's clearly not something many want or believe to be feasible.
2. India is simply not as economically developed as the other nations, and so it has fairly lax regulatory laws which make free trade difficult, if not impossible.
3. India is diverging from sensible political thought. While the four included in CANZUK are liberal democracies, President Modi is actively pursuing discriminatory policies against Muslims, even depriving them of their citizenship. That's not something which the other four could accept.