There are a lot of old threads relevant to this...
FWIW, In the 1920s the standard advice was 9 months.
In the 1950s the standard advice was 4 weeks (!)
I'll try to explain as I understand it.
A small minority of babies will start to run low on iron stores in their bodies at around 6 months. Iron is essential for growth, esp. brain growth.
It used to be thought that to be on the safe side, start solids between 4 & 6 months, so that baby was getting steady dietary iron by 6 months. The UK govt./Dept. of Health took this perspective. Esp. since breastmilk is quite "low" in iron. In fact, 1970s advice was that it was "bad" to keep breastfeeding after 6 months because of the low iron in breastmilk.
Current research is that while breastmilk is low in iron, it's not like iron in the food you or I eat; what little is there is fully available to the baby. So breastmilk is perfectly adequate in iron after all, at least until 6 months, often longer. In the meantime, a lot of studies show a link between early solids (esp. before 15 weeks) and lifelong bad health conditions, like asthma. It's generally thought that the earlier solids are introduced, the more likely they are to cause an allergic type condition.
So... About ten years ago the World Health Organisation took the view that solids simply aren't necessary before 6 months, and they may pose a risk, so don't introduce any sooner than strictly necessary (ie., 6 months). The Uk has only just got around to the WHO guidelines in last 2.5 years.
hth