Safety is a big one. Dumbasss do dumb things and sue the OEMs and pretty soon you have 6 safety sensors to satisfy to start a machine. Technology gets better is another reason, if company A built a 1845C type SSL with that old tech, very reliable but not refined in any current way and Company B introduces a quieter, more powerful, pilot controlled machine that is easier to run and that lifts more weight and even though machine A is probably more dependable, the B machine is much nicer to run so they sell more. Emissions is another forced criteria to meet, which results in newer models to be built that allows for the emissions requirements to be added. For me, if they still built an 1840-45C machine, I wouldn't buy one new (and I have had 6 of them over the years when you could still buy them new). The newer machines do almost everything better. The only advantage to a vintage machine is anyone could work on them and they don't have near as much to go wrong, but the newer machines are way more productive.