Because it says so. Read again.
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UPDATE: It has been brought to our attention that the following method will not work for some devices running stock ROMs because their manufacturers have made the ridiculous decisions of either using their own proprietary format for their boot animations (we’re talking about you, Samsung!), or choosing a non-standard path for the bootanimation.zip file and removing the ability to override it by copying a new one to /data/local (that would be you, HTC!).
In case of the proprietary format used by Samsung, there is little we can do. However, the HTC devices use the same format but at a different path i.e. /system/customize/resource/ instead of the usual /system/media. This means users with rooted and S-OFF HTC devices can still replace the original one using the following method, by simply changing /system/media to /system/customize/resource/ but those without rooted and S-OFF devices are simply out of luck, as the /data/local method will not work on such phones.