There are 2 main reasons – structural suitability (usually) and lack of space and height as most bi folding door extensions are a flat roof single storey extension. It is possible of course but just needs some planning for the correct support.
Sadly for us, we have a weekly conversations with people who have just had beautiful outward opening bi-folding doors fitted on a single storey extension, often with 95cm deep doors and often with a soldier course of bricks in the steel above the glass with little or no brickwork above. The awning ideally needs a 14 degree pitch to be effective in rain and don’t forget the cover (fabric) is not the lowest part of the awning the arms hang lower.
So often is the fact overlooked that a descent sized self supported awning required some fairly hefty fixing plates and strustural support at the back for the main cassette and this is just not available on single storey buildings so fairly long spreader plates have to be used to spread the weight down the lower courses where there is some strength.
It is certainly not impossible but there does need to be some consultation ideally to ensure either more height is allowed for or the steel beam is manufactured in such a way to take the awning weight safely.
The lack of brickwork courses above the bi folding doors usually means there is not sufficient strength in compression to take the weight of a retractable awning. If there is sufficient side space of course you can fit using spreader brackets and then have the chance to lift the awning up higher on the brackets to at least get a half descent degree of pitch and also avoid hitting the door sections when opened.
It is also possible to use one of the more modern retractable roof systems available such as the Pergola or Pergotex systems where a framework is used offering far higher wind and rain protection and much larger projections. This system can be fitted with a rear frame support negating the requirement to use any of the building in any way.
The image below shows a Markilux Pergola fitted directly above a wide sliding door installation however this particular building does have an apex shape and therefore some substance in the structural properties of the brickwork this is fitted too.