The difference with 32 bit software is that there's nothing forcing them to do it. With something like a port, there are physical limitations to the device, so abandoning older technologies at least gets you something in terms of form factor.

With 32 bit support, these exact same CPUs could easily still run 32 bit software as they have for years. The tradeoff I guess is that it makes the OS a bit easier to support from Apple's point of view. The advantage to the user is not clear.