The same reason people invest in big companies like Apple and Google, rather than dealing solely in penny stocks.
It is true that they could get lucky and spend a third of the budget they'd spend on a Trek series and have a successful new sitcom that makes the same or larger profit margin than a Trek show would, but there is also a MUCH higher risk that it will fail.
If you invest into more money into an established franchise with a big fan-base that would watch the show it has a much lower risk of failing. It also has the added benefit, that whether the show is a success or failure, they will have more of an opportunity to make money on recurring DVD/Bluray/Digital sales as well as merchandising rights.
Any executive in charge of green-lighting a show is going to have to explain how episodes #701-? are going to outdraw Enterprise which ended its run with less than two million viewers a week. They're also going to have to explain why the franchise suffered dwindling ratings starting with Deep Space Nine and why there was little growth in the box office numbers for Star Trek Into Darkness
in comparison to Star Trek 2009
Trek may have worn out its small screen welcome with the over-saturation of the 90's and high production costs.
I think eventually it will return, nothing stays dead forever in Hollywood. But I think the product will be so different from the Berman shows that it will be unrecognizable. Which may end up being a bigger blow to those who want Trek back than no show at all.