1. Short Lifespans: These stars are incredibly massive and hot, burning through their hydrogen fuel at a furious pace. Consequently, their main sequence lifespans are extremely short, on the order of a few million years for O stars and tens of millions of years for B stars. This contrasts with our Sun, which will spend about 10 billion years on the main sequence.

2. Rapid Evolution: Due to their high mass, O and B stars evolve quickly. They rapidly exhaust their hydrogen fuel, expand into giants, and eventually explode as supernovae. This rapid evolution reduces the time they spend as main sequence stars.

3. Formation Rate: While massive stars form more frequently than expected due to gravitational collapse of gas clouds, they represent only a small fraction of the stars that form in a galaxy.

4. Metallicity: The formation of massive stars is more common in regions with higher metallicity (the presence of heavier elements than hydrogen and helium). While metallicity can vary across a galaxy, the overall proportion of massive stars remains relatively low compared to less massive stars like our Sun.

5. Observational Bias: While massive stars are rare, their brightness makes them easier to observe. This means we may overestimate their number compared to smaller, fainter stars that are harder to detect.

In summary, the combination of short lifespans, rapid evolution, and relative rarity in terms of formation rates makes O and B main sequence stars uncommon compared to other types of stars.