perhaps it has something to do with the fact that it was easier to port existing compilers and perhaps even designing the first microarchitecture to support it and give decent performances without having to wait too much and loose competitive advantage.<p>I guess that 32bit mode could use most of the same microarchitecture and thus guarantee that during the transition period people would still buy those new chips.<p>Itanium was a good example of such a strategy that failed. However there might have been other reasons as well.