You can't just release a 64 bit chip and expect a full stack of 64-bit clean software to be available on day 1. Indeed, last time I checked Dalvik wasn't fully 64-bit clean. Releasing earlier than you need it ensures that by the time you do need it, the software is ready to go.
Also, besides the correct points made in sister comments, there is the issue that virtual address space is used for things other than simply mapping physical memory. There was a time, for example, when Linux mapped all of physical memory into the kernel's virtual address space. This was simple. These days, on 32-bit systems, it splits the 4GB address space into 3GB for the user and 1GB for the kernel, and maps physical memory in and out of a 128MB window in kernel space, as needed. This is obviously more complicated.
Also, besides the correct points made in sister comments, there is the issue that virtual address space is used for things other than simply mapping physical memory. There was a time, for example, when Linux mapped all of physical memory into the kernel's virtual address space. This was simple. These days, on 32-bit systems, it splits the 4GB address space into 3GB for the user and 1GB for the kernel, and maps physical memory in and out of a 128MB window in kernel space, as needed. This is obviously more complicated.