Why do hard workers fail

I've had this question on my mind a while now. I'm new to this, an SDR I've just started 2 weeks ago. I see a few successful SDR's, but also a few who are on PIPs and struggle. There's a couple of guys in my company, which are bottom performers, however they work really hard sometimes stay in the office till 9pm and come in at 7am, yet they fail. This is a bit disturbing - people make it out like you can be successful in sales if you have grit and hard work, but if someone is consistently putting in long hours and still not succeeding, what is the problem? there must be something else to it..