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 well saved money (*1) to make the CPU run according to whatever the video needed. In real world usage it didn't matter that an Apple II was running at (roughly) 1.024 MHz to satisfy instead of 1 MHz, or a C64 at 1.023 (NTSC) or 0.985 (PAL) (*2). Or that many Z80 systems did utilize less than the 4 MHz the CPU and other components were designed for.
Were talking (usually) less than 10% difference from some hypothetical speed, so way below anything noticeable beyond some benchmarks. Not to mention that these were usually way more influenced by software implementation details, like the BASIC used, than the hardware itself.
Sure, speed was a criteria, but only one among many others, like RAM size, screen resolution, ability to show 80 columns or colour.
And, eventually the killer one: PRICE
So what good would it be for a manufacturer in such a price sensitive area, as home computers were, to add hardware to archive an effect barely noticeable, but increasing price quite drastic? (*3)
Building computers is a business to make money, not a race for the best machine - that is only done if money doesn't matter (but performance does).
The ZX81 is maybe the most prominent counter example here - it wasted 60+% performance to save on video hardware - and people bought it like sliced bread.
*1 - With synchronized access windows a concurrent access simply never happened, so no need to add hardware for dual access, thus less cost. It's like with waste (refusal) management - best waste management is to not have waste in the first place :))
*2 - It did ofc, matter when porting tight knitted software between either, but that's beyond the point.
*3 - Keep in mind, counter price of any hardware component added is usually at least four times that components price. So adding 25 USD in logic and RAM interface results in 100 USD higher sales price.