I'll try and answer this question based on what I know. I do warn I'm not an expert on this subject and there are probably better sites on the Internet that deal with these subjects. I however am confident that what I do say can be cross-checked, given enough sourcing research. This being said, let's start from the top.
... until some years prior OSX ... only could be run in PowerPC CPU's ...
It is true, until Tiger (version 10.4) came along. However, it is not a true statement as is. You could "emulate" the PowerPC architecture on an x86-64 processor (as you would if emulating an old game console or arcade system). I doubt that would be feasible.
I do remember there was an attempt to make Panther (version 10.3) work with x86-64 processors, that would involve rewriting most of what existed on OSX at the time.
... but that changed when Apple adopted Intel CPU's opening the possibilities of installing OSX in PC's
True and false. What changed was the fact OSX applications could now run with an x86-64 architecture, besides a PowerPC one. Running on an unmodded system is near impossible, because while the processor changed to the one most PC's use, the hardware has not.
The greatest roadblock has been passed though, and as such, you can install special versions of OSX on some PC's. What is the difference between a special version of OSX and a regular one?
Special versions of OSX have modifications to the underlying structure of the operating system, in order to accomodate the differences at hardware level. One of those differences is the fact OSX doesn't use the BIOS most computers use and instead resorts to the EFI system to load the devices to the system.
By some computers I mean that, even with the processor door open, there are few devices Apple introduces to their own computers. As such, obtaining the kexts or the EFI strings (i.e. drivers) for those devices is a hard process. As such, most computers fail on this part.
The original kernel for OSX requires the SSE3 instruction set, but there are some kernels that manage to emulate the functions provided by the SSE3 instruction set using the older SSE2. This isn't a big problem, as most modern processors have the SSE3 instruction set and as such is relevant only in very old processors.
Why did they make it so difficult?
Apple, understandingly, doesn't want people to install OSX on computers they don't make. They wouldn't be able to support users. Also, it cuts costs, reduces bugs and overall gives a better user experience to have a very standardized and uniform product line.
How they make it so difficult?
As stated before, it isn't difficult; you need some research beforehand and you can get a working OSX installation in no time. Doesn't mean it works fully, but you can move the mouse click on items and the such.