Why is Apple Mac not a popular gaming platform?

That is an easy one to answer.

Macs are far more expensive than PC builds as are handicaped by near abscent upgradeability.

Because they're computers and not toys.😉

Macs are indeed popular gaming platforms, as evidenced by the many games available for them. Macs are not as popular as Windows system for gaming, particularly games that involve high levels of animation, because Macs don't have the ultra-high-performance graphics cards many gamers demand, though in many cases those cards don't really add anything other than status and bragging rights for the user. Many dedicated gamers also prefer to build up their own systems, choosing all the parts to suit themselves, something obviously not possible with a Mac.

Regards.