The original IA64 parts couldn't run x86 code worth a crap, and so not only did you have to change everything to use the new 64-bit functionally, you had to change everything before the device was even useful. That's a much bigger hurdle to clear.
Assuming ARMv8 processors run ARMv7 code relatively well outside of AArch64 mode, then you could deploy quite a lot of hardware with ARMv8, and only move to 64-bit when needed. I think that was much of the allure of x86-64. Many people ran 32-bit OSes on 64-bit devices for quite awhile, until the 64-bit software matured.
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