A little background may be helpful in understanding why this is the case. In the very early days of the automotive industry the Cooling system only consisted of a radiator and a water pump. The understanding that water expanded as it heats up was considered and thus the radiator itself was never really filled to the top. Boiling point, etc. was all controlled by a pressure cap on the top of the radiator. If the engine got too hot the cap would release pressure by simply venting water to the ground. Thus it was not uncommon to have to check and refill the radiator from time to time.
The next generation cars saw the introduction of the first "Overflow" tank. The overflow tank was used to collect the water that was expelled from the pressure cap when the engine got too hot. The concept was that, as the engine cooled down, the the system would automatically recover the fluid in the overflow tank and all would be fine for the next time.
As emissions standards were tightened and tightened over the years the system further evolved by moving the location of the pressure cap from the radiator to what is now often called an expansion tank.
These tanks, having been called "header tanks", "overflow tanks" and "expansion tanks" now had to withstand all the same pressures of the radiator and this is the main reason for all the attention in the case of the MINI. The stock tank in the MINI is a molded plastic affair that while made with the latest technologies has simply proved to not be so reliable. While it is believed that the latest factory versions no longer have the problem, a number of aftermarket metals version have grown in popularity.