Honestly, I think it's just a matter of Games Workshop doing a Nokia - the company became successful focusing on creating good products then changed to focusing on making money. (Fun fact - a Nokia engineer actually created a prototype smartphone before LG or Apple, but the money guys felt sticking with the old super-profitable ones was the way to win).

The biggest advantage over 40k, from my perspective, is that Grimdark Future uses turns, I.e. 'my unit, your unit' instead of 'my whole army, your whole army'. Each side moves each unit once per round, but you just take turns moving units getting there.

Grimdark Future Firefight is my favourite though - instead of huge battles, you take smaller teams. It feels more cinematic. Units don't die as easily and you can pin a unit with suppressive fire. If you attack someone on the edge of a platform and knock them down, you can boot them over the edge. And best of all, it's fast - games can be wrapped up in under an hour easily.

Oh, almost forgot - you don't actually have to buy anything either. The standard version of the game, with army lists for all armies, is free. The army building app is free. You can buy advanced rules if you want more nitty gritty, but you're literally good to go right now.