*because it isn't technically 8-bit.
First, 8-bit is a sub-category of Chiptune, and the GBA doesn't use 8-bit, that's for sure. An article on Uzerfriendly details how the soundtrack was creative and highly impressive for the time. The article claims that: "...before the release of these two games, the handheld Pokemon games were comprised of 8-bit sounds and medleys..." alluding to the fact that the GBA did not use that technology.
My fourth grade teacher will kill me, but I'm using Wikipedia as a source.
Sound: Dual 8-bit DAC for stereo sound (called Direct Sound), plus all legacy channels from Game Boy. The new DACs can be used to play back streams of wave data, or can be used to output multiple wave samples processed/mixed in software by the CPU.
Apparently, it uses a dual 8-bit system. I have no clue how that works; not a tech guy. But, since it isn't your standared 8-bit system, I can safely assume it to be Chiptune.