Everyone runs to the pyramid - it was explained the greatest fighters in the realms are attracted to the battle like moths to a flame.

Shang Tsung is actually physically aging into an old man. His youth was restored artificially back in MKII, he truly is a decrepit old man, and this showed in MKA when he temporarily lost his power. That's why his hair changed from black to gray, he also got a bunch of wrinkles, etc. This also gave the designers an excuse to show Shang Tsung in his MK1 look, thus making the story come complete circle from the early MK games to now.

Thanks and why Blaze become boss does he most powerful character in storyline now or it's just change because Onaga was already boss in MKDeCeption