Cat/Mitsu went to the K21/25 engine because Mitsubishi could not meet the Tier 3 standards by the time period set out by the Fed. I'm sure you will eventually see a redesigned 4G63/64 back in the Mitsi/Cat product in the future. Why not use your own engine? As far as why OEM's do different things to their lift trucks, the answer is easy: price. American companies build products under strict price guidlines, in other words a price is set from the start of the project and a piece of equipment is built around that price. That is the unfortunate American way. The European way is to design a piece of equipment to do a job, then price the equipment after development, whatever the cost may be. Example: Linde, Jungheinrich, Still forklifts. 30 - 40% higher up front cost than their American counterparts.
So, when you see an "American" forklift company change vendors or change design of components it is for only one reason: price. I think we can thank Walmart for this ridiculous American way of thinking, cheaper is better.