Kangaroos hop because they can store a lot of energy in there Achilles tendon and use that to propel them forward without using much energy. It's good for covering long distances over flat terrain while using as little energy as possible , which is good for the Australian outback as they hop between small patches of vegitation separated by miles of desert. It's not that good for ambushing or quickly chasing prey in a rainforest like the t rex is probably doing.

There's also the issue of scale, a t rex's Achilles tendon would have to be stronger then steel cable and as stretchy as rubber to store and re use that much kinetic force from its weight.