Tourist inflow is dependent on two things one is the enough places for the tourist to visit and the comfort level on how tourist can visit. Now lets put this answer in this context - we see India is one of the oldest civilization so it has a lot of places of historical importance, it is the origin of four big religion i.e. Hinduism, Sikhism, Buddhism and Jainism so it has lot of potential for religious tourism, it has a large coast line, great Himalayas, Thar Desert and lot of big rivers so has lot of natural places to explore so we can say country has lot of places which can be explored by the tourist but it lacks infrastructure i.e. Air connectivity is weak, not all places has good hotels, connectivity and other infrastructure to reach many natural places is not upto the mark. Which is the main reason why country receive less tourist then its potential.
Answer to your second part is fairly simple i.e. improvement of the infrastructure would attract more tourist to India.