Why are there not many stars on the upper main sequence?

There are fewer stars on the upper main sequence because these stars have shorter lifespans compared to their lower-mass counterparts. High-mass stars burn through their nuclear fuel quickly, exiting the main sequence in a relatively short time, often in just a few million years. In contrast, lower-mass stars can remain on the main sequence for billions of years, leading to a greater abundance of them. Consequently, the upper main sequence is less populated due to the rapid evolution of high-mass stars.