Seriously I really hate that game. It's a very blatant ripoff of TF2 and the developers are SJW's. I also hate how generic the games aesthetic is. It's just the same Sci-fi FPS look we see in every game although I haven't played it the game doesn't seem the game adds anything new to the table gameplay wise. So in general you'd rather be better off playing TF2.
So why do people like it again?
...Dont do the "SJW" thing. Seriously. No matter who uses that term, and no matter the reason, it never paints that person in a good light.
The internet pulls that derpage enough as it is, dont need even MORE of it, and it has literally nothing to do with the game. That's one term that needs to go the way of the dodo. The fact that it isnt already long-dead baffles me. I know the collective internet is about as bright as a box of hair, but that one is getting ridiculous. They could at least come up with some new stupid thing to focus on and whine about.
AHEM. Anyway....
As for the game itself, it's a few things:
1. It's Blizzard. Many people will buy almost anything Blizzard spits out, even despite the link to Activision. They are freaking legends in the industry, with a legacy that reaches all the way back to the SNES era. Very, very few other developers can even hope to match that level of sheer renown and status.
2. Bright, colorful, memorable characters, well animated, in a bright and colorful world. It's pretty hard to do this in such a way that makes it memorable, in game design. But they pulled it off. It's a subjective thing, but many would agree that they did it right. If anything, TF2's characters are a bit more generic than these guys. That game was conceived in a bit of a different era, and in this day and age, it really shows.
3. SHOOTY BANG BANG
4. It's designed for e-sports type play, which means a really extreme focus on a specific sort of competition that encourages people to form permanent teams. TF2, on the other hand, is more designed for chaotic multiplayer. Believe me, there IS a difference between those two concepts, and it's a big one. It'd take way too long for me to describe it though.
5. As much as I hate to say this, I do suspect that some players have reached the point of genuine addiction. The lootboxes and all. Note that TF2 can do this too, albeit with different tactics. But Activision is one of the two most evil publishers, so Overwatch is more effective at that type of treachery.
6. There's a bit more depth to it. TF2's combat is a bit on the simplistic side. And yes, I've played it, and Overwatch, before. Overwatch however goes quite a bit further with the character gameplay designs, giving them more abilities, and more importantly, really VARIED abilities. TF2 never did that very well. Overwatch does it REALLY well. I dont like Activision-Blizzard one bit, but I gotta hand it to the devs: They did one hell of an amazing job on this aspect of the game. That's not too surprising though. Blizzard is considered to be a legendary developer for a reason, even if they have started to tumble hard in the last few months as Activision's corruption of them enters it's final phase.
7. The e-sports style of focus means that things such as tournaments are broadcasted alot, giving the game alot of added visibility and getting consumers interested. Activision-Blizzard may be evil, but they are a very active sort of evil. They do push those tourneys and such, and the Blizzard name alone adds alot of extra muscle to it. For Valve, TF2 kinda just sits on the horizon for them, watching from a distance. They stopped focusing on it long ago, and shifted their full focus to things like Dota instead (makes sense, with how that exploded. They would have been utter fools if they didnt focus on Dota like they have). Not to mention that Valve is an alarmingly lazy group in alot of ways. I say this as a very long-time Steam user.
8. Overwatch is new. TF2 is old. That one is an incredibly simple point, but when it comes to multiplayer games, it's also one of the biggest points. The longer something has been out, the further it inches towards becoming a dead product. That TF2 has lasted THIS long is nothing short of amazing. But that age means that it has little chance against such a juggernaut.
9. Youtubers and Twitch streamers scream about it alot. Like, ALOT. That's even more coverage and more consumers tempted towards a purchase.
10. In a way, the things that have been done with the characters and such also affect it. When people think of characters from Overwatch, they will then specifically think of Overwatch, the game. Makes sense, right? But when people see TF2 characters, that may not be the case. They may think of Garry's Mod, or the 5 quintillion videos/skits/memes/whatever that those characters have appeared in. Valve did a good thing, I think, in allowing basically anyone to use their characters for basically anything. But this also shifts focus away from the main product by associating it with too many other things. Which didnt matter too much when TF2 was alone, but now that it has direct competition, they're starting to feel the side effects of it.
I could probably keep this up for awhile, but I think you get the point. I will always repeat the idea that gameplay is more important than anything else... but gameplay alone isnt what drives sales or pulls in attention from consumers and actually holds it there. There's so much more to it than just that. Much to my constant annoyance.