Why go to the BREN?
Lets look at a basic fact. If you take the VZ-58's history into account you find it has been a trouble free assault weapon. Now, look at the BREN. Here we have a new weapon using a NATO round and it has a major problem with being reliable. Now, why spend big money the Czech Republic doesn't have when they could produce the VZ-58 in 5.56, as already being done and call it a day. Add a gob of 1913 rails and more plastic body parts and you got a modern firearm.
I understand why the US military is looking into a new battle rifle, all the M-16 needs is a piston system and a 6.8 SPC round, they want something with more long range power and also more reliable. The Czech's are not in that situation and therefore have no real need to change from what works to what does not.