Because they're different types of players filling different roles. They are forwards but that's about all they have in common. It's like asking "Why bring in Benteke when we already have Agbonlahor?" Different attributes to fill different holes in the squad. There's very little that's similar about Bowery and Helenius. Bowery is a big powerful guy, good at holding up the ball and battling with big defenders, handy for taking the ball down to the corner flag when trying to hold on to a lead. Helenius is tall but better with the ball at his feet than in the air, can play in the hole to lay off balls to the main man or run onto rebounds. They will develop differently and into different types of players.
I think Kozak was brought in to be a long term replacement for Benteke and I think he will play in a similar role to Benteke. He is still getting used to his teammates and the league and isn't ready to be that guy just yet, but we all know Benteke will be off next year (much as we'd all like him to stay). Kozak has a year to bed in with the squad and grow into that role. He stepped into the shoes a bit early due to the injury and he isn't the finished article yet, but it took Benteke several games to find his feet in the league too. My expectation is that Kozak spends this year bedding in, getting used to the league, getting game time up top with Benteke or off the bench, occasional starts for when Benteke's tired or injured or suspended (the way he throws elbows around he'll be bound to miss a match or two), and he'll be ready to lead the line next season.
Helenius and Bowery will probably take a little longer to break into the first team. Bowery might never get there, to be honest, he may never be anything more than a squad player, but the money we spent on these two players, around 1.3m, would not have gotten us too far in the market -- even if you combined that with the Kozak money, for a combined total of about 6m. We would not have been able to bring in the AM we all crave for that kind of money, at least not one ready to go straight into the first team. In Helenius, I think we have a guy who has a very bright future, but that future might be a couple of seasons off still.
I thought it was smart to bring Kozak in this year and give him a whole season to bed in, and I think it will pay off in the long run.
This is well put Heid3ster, probably the plan is something like this.
I wish I was as sure about Kozak's abilities as you. So far, I'm not. But will give him much more time. When Benteke is back, we'll forget about this guy for a while, which may be good for his development. Sitting on the bench for a year probably isn't though.