They have a huge surface area over which to radiate their energy. The white dwarfs are at the opposite end in the lower left of the diagram. They have a small surface area and so the sum of the total radiated energy is small. Luminous stars are easier to observe because they can be seen from great distances away but they are rarer in the galaxy. They tend to reside in the top half of the H-R diagram.
Faint stars are harder to see but are more common in the galaxy.
They tend to reside in the bottom half of the H-R diagram. This method is called spectroscopic parallax because a distance is found from knowledge of a star's spectral type. Distances for stars too far away to show a detectable trigonometric parallax are found this way. Use a calibrated main sequence to get the star's luminosity. The Hyades cluster in the Taurus constellation is the standard calibrator.