Answer: Steel alloy beams are harder and stronger than pure iron beams. A small amount of carbon makes the steel stronger.
Steel is a well known and commonly used alloy (a mixture of several metals) to build a high rise building because it can be around 1000 times stronger than iron in its pure form. It is made from iron and a small amount of carbon (usually between 0.2% and 2.0% by weight) that makes it stronger and lighter.
Pure iron beams, on the other hand, has the bigger quantity of carbon that makes its structure weak and fragile, thus not fit for high rise building.