Direct injection is actually a fairly old technology, with the 1955 Mercedes 300SL using direct injection, and diesels using DI for quite awhile. Gasoline DI only became mainstream around around the early 2000's, and many of these early engines had issues with carbon buildups forming on the intake valves; an issue that was rather uncommon for other EFI engines. The reason for this is fairly technical, but I'll shorten it for clarity and provide a link that explains it in more detail. Basically, combustion byproducts get recycled into the intake tract through the PCV valve, and will cling to the intake valves on their way into the combustion chamber. With port fuel injection, fuel is constantly moving over the valves, in effect "washing" the valves and preventing carbon form building up too much. With DI, the absence of this fuel allows the carbon to accumulate, and if left unchecked, can prevent the valves from seating properly, causing a number of issues.