2 reasons.
Battlefield 4 came out a year before so people didn't want to spend $60-$120 on another battlefield game so soon.
Battlefield 4 fucking sucked when it released, buggy as hell and didn't even work for a lot of people. People were outraged and felt ripped off. A year later when Hardline released Battlefield 4 was still a mess so people were angry at EA and saw the release of Hardline as a blatant cash grab. Why would they buy another game from the same company that still haven't fixed their old game? And if they have the resources to make another game why the fuck haven't they fixed their old game people already payed for?
There's other excuses people parrot all the time, cops and robbers reskin, basically an expansion pack of battlefield 4 blah blah blah. None of them make sense if you played both games. Hard line was more infantry focused with no tanks and no attack choppers/planes which didn't appeal to some people but mostly there was a lot of anger at EA at the time. Which is too bad because I thought Hardline was an awesome game, I had more fun playing it than Battlefield 4.