In the world of Pokémon, Mega Evolution was not necessarily an improvement. Garchomp was one such example. Players always felt that regular Garchomp was much easier to use than its Mega Evolution, making it one of the few Mega Evolutions in the Pokémon games that amounted to a downgrade.
That was not to say Mega Evolution was weak. After all, it was a Pokémon's final evolution and brought a considerable increase in base stats. Calling it weak would be denying the obvious. However, Mega Evolution also changed a Pokémon, and changes came with drawbacks. For Pokémon like Garchomp, the drawbacks before Mega Evolution were much smaller than those afterward.
Every generation had its king, but every generation had Garchomp.
Anyone who played competitive Pokémon simply could not avoid Garchomp. It had been strong across the board ever since its debut in Generation IV. Whether you played six-on-six, bring-six-pick-three, or bring-six-pick-four, there would always come a point when you added Garchomp to your team, even though I admittedly never managed to use it very well.
Garchomp looked cool (debatable; I thought it lost a lot of its visual appeal once it lost its Generation IV pose), had excellent typing (its dual STAB offered extremely strong offensive coverage, and its resistances were good too, with only three hard-to-avoid weaknesses: Fairy, Dragon, and Ice), boasted pseudo-legendary stats (a favorite among people who liked overwhelming opponents with base stats, though that advantage shrank as more lesser legendaries appeared), and had a sensible stat distribution (that obnoxious base 102 Speed put Garchomp on the grudges list of countless base 100 Speed Pokémon, while its high HP and middling Special Attack allowed it to maintain reliable bulk without sacrificing a high Attack stat; most Hidden Power Ice attacks could not knock it out in one hit).