I think they're using steel frames this time so that its easier to change things around on the set
The main reasons to use steel frames are cost, speed of construction, resilience to the elements.
I don't know what they intent to build the façades of the street front from, but usually it's a marine-grade ply (wood that's treated to survive in watery environments), and then detailed with a type of plaster which they can make look like brick or stone. You'll see similar construction techniques in themeparks.
I doubt they'll intend to change things once its constructed - although you're right - its probably much easier with steel frame.
The bigger question is, why did they built the 30 year old street in brick? The EastEnders lot is the same age, and it's a steel/ply job.