Here's why:

* O and B stars are massive and hot: They have much higher temperatures and masses than our Sun.

* Short lifespan: Due to their high mass, they burn through their fuel very quickly, leading to short lifespans (only a few million years).

* Rapid evolution: They evolve quickly and don't spend much time as main-sequence stars, making them less likely to be observed.

In contrast, stars like our Sun (G-type) are much more common. They are smaller, cooler, and have longer lifespans, allowing them to remain as main-sequence stars for billions of years.

Here's a general breakdown of the frequency of stars in different spectral classes:

* O: Very rare

* B: Rare

* A: Less common

* F: More common

* G: Common

* K: Common

* M: Very common

It's important to note that the actual frequency of stars in different spectral classes can vary slightly depending on the region of the galaxy being studied.