AFAIK you needed to pay a license fee to write programs using Jazelle instructions (so you needed to weigh whether the speedup of Jazelle was cheaper than just buying a more powerful CPU), and the instruction set itself was also secret, requiring an NDA to get any documentation (so no open source software could use it, and no open toolchains supported it).
I remember being very disappointed when I found out about that
That’s a blast from the past; native Java bytecode! Did anyone actually use that? Some J2ME phones maybe? Is there a more relevant example?