There are plenty of factors as to why studios hesitate to back new properties. But the one that's the most difficult to argue with - from within the industry - is rights.
Studios already own the rights to thousands of properties from the last 70 years, so it's easier to reboot something old and owned by the studio than create something new - and having to jump through fresh legal hoop$ to determine how residuals will be split for the next 50 years. Union contracts and the evolution of the industry (broadcast TV, cable rights, internet, streaming both at home and on mobile devices) is to blame for that.
Studios who have a majority stake (and full control) in properties like MacGuyver, Quantum Leap, Magnum P.I., Fatal Attraction, Fresh Prince Of Bel-Air, and on, and on, and on, face less risks, and that makes it easier. Whether that makes those reboots any good is arguable.
There are plenty of original properties sitting on shelves, not getting any attention. The lack of talented filmmakers and creative executives willing to stick their necks out for such properties (see Gary Kurtz) is one of the reasons.