I think it is because of three main reasons... one is the weight used.. second one is in making the structure.. in steel it is easy to make when compared to wood... the third is l conductivity.. steel can transfer heat outside and inside.. but wood cannot.. woods are insulators to heat.. if the climate is cool, it won't take heat from outside to inside and vise-versa...

The last is, wood can be easily affected by various factors like climate, termites etc.. and it can be easily fired too..

Actually it's more of a shift from metals to plastics (read fiber glass composites)

Hello Izran Kamal, I was a bit surprised to read about a shift from wood to metal for window frame materials. I wonder on what this perception was built on. Commenting from within (Germany), my impresion is Samson Rwawiire is right. A Major concern in German as well as elsewhere in Europe is heat conductivity , meaning heat loss. Wood Frames require frequent surface maintenance which many house owners try to aviod. High Quality Aluminium window Frames are good but too expensive. I haven't seen steel window Frames (only for fire-proof steel doors in technical Environment). Since the Quality of Plastics ensures no colour Change over many years nowadays, the much cheaper 'Plastics window frames', which usually have steel core elements for stiffness and stability improvement, gain in application. The reason for the lower Price is most likely to be found in lower production costs as Plastics Extrusion Technology is widely developed. Make Frames of Wood or metals is more like to be more expensive because of higher Labor costs for more complex production.

Regarding the comment by Ram M S, I don't think, that weight is an Argument, but the strength of the steel still is. The flexibility could be e.g. for welded structures, but the conductivity is not. It is not appreciated to have higher heat loss through the window Frame and usually we don't have Problems with termites either (at least in Germany). Wood is a good insulator as he stated, that is why still many People choose Wood because they prefer natural materials. We are looking for the best heat insulating materials and Frames designs, that is why the plastic Frames are so interesting.

An approach that was developed in South Africa recently (Agriligna (Pty) Ltd) is to use wood-polymer composites (WPC) in the extrusion of window and door frames. Apart from the positive economics compared to aluminium, steel and polymers, the maintenance of such frames is a major advantage.