Well, depends on the definition of dual port. After all, all 9918ff based machines can as well be classified as dual ported. Similar PC graphic cards, like VGA.
Beside that, the most most important reason was No need to do so.
Early 8 bit CPUs were not only slow enough (compared to RAM) to allow interleaved video and CPU access on a fixed schedule, it as ...
The IBM PS/2 model 30 (and it's variant, model 25) use 64K of VRAM in their MCGA video system. They manage to display the 256-color VGA graphics mode (requiring around 12.6MB/s streaming to the monitor) with an 8-bit data bus, whereas the VGA card requires a 32-bit data bus with its single-ported data RAM to provide enough bandwidth.
The video RAM chips are ...
Given packaged component costs at the time, for a given performance target, it was cheaper to time division multiplex either a wider or faster memory bus than to procure dual ported chips (much lower volume and/or higher pin count packages, synchronization circuit costs. etc).
Pins were not free.