When planes approach they have to maneuver between very high Himalayan mountain peaks, as high as 18.000 ft. The runway itself is only 1,980m in length and thus makes it even more difficult for the pilots.
Very few pilots can make this difficult approach.
From the wiki: Flights to and from Paro are allowed under visual meteorological conditions only and are restricted to daylight hours from sunrise to sunset.
You might also like to see Mumbai aerial view - pictures on the way out of Mumbai