It's all a part of Moore's Law, and to some extent, rampant capitalist behaviour.
On one hand, as new technology is developed, new software is designed to take advantage of said new technology, which usually includes performance enhancements unavailable in older technology. Thus, a computer that was greased lightning yesterday is molasses in January today. Part of this is technology based, but more than part of it is also planned obsolescence. There is absolutely nothing you can't do with a PPC Mac, but it depends on the software you ask it to run to do this task. If you keep period-appropriate software on your machine, then your performance will remain relatively similar to when it was new. If you continually ask more of it, however, it will fall further and further behind. There's no money in you keeping a computer for ten years, compared with someone who upgrades every other product cycle, so software and hardware design are tailored to those people's needs, not those of us who use older machines. Thus, support slowly erodes for older technology, even though it's perfectly capable equipment for certain tasks. It's all a question of money.
Case in point; if I were so inclined, I could still write books on a Macintosh SE. I'd have to live without some modern conveniences, but it could be done. Try finding support for that computer. 🙂