> Why has it taken so long to get the Windows ecosystem fully on ARM?

Windows has been through several ARM phases.

The first one being Windows CE 2.0 in 1997. Win CE was used in the Dreamcast and the Gizmondo handheld.

Lately confusion arose when people started calling arm64/AArch64 for just "arm".

Does Windows CE really count as Windows though? It’s a cut down separate thing to proper Windows really (although my view that it’s not Windows if it isn’t providing the Win16/32 API isn’t one Microsoft themselves share).

Also, the Dreamcast uses a Hitachi SH4, not ARM.