I spent some time reflecting on this, and I think it's because what the bottleneck is has changed over the years. For a while, I/O was the real challenge for servers. But hard drives have improved a ton, and many-drive systems have really improved a lot. Memory had its day, but the price has plummeted.
Today, the limiting factor everyone is focusing on is core density, and cores generally produce more heat than any other component aside from high-end graphics and displays.
Of course, power consumption and thus heat has been continually improving, but not by orders of magnitude. Core density, on the other hand, has been going up by a few orders.
As a result, you've got to move more heat, and because density is what's going up, you've also impaired airflow.
Take this with a grain of salt though, it's just speculation based on what I've seen & heard.
Today, the limiting factor everyone is focusing on is core density, and cores generally produce more heat than any other component aside from high-end graphics and displays.
Of course, power consumption and thus heat has been continually improving, but not by orders of magnitude. Core density, on the other hand, has been going up by a few orders.
As a result, you've got to move more heat, and because density is what's going up, you've also impaired airflow.
Take this with a grain of salt though, it's just speculation based on what I've seen & heard.