The reason the Wii doesn't play dvd video discs is because there is no software on the Wii to be able to play them.
The Wii is capable of playing dvd video discs as a programmer found out after he made some software to be able to do it but it only worked on modded consoles.
The capabilities are there just no official Nintendo software support for this.
Nintendo may introduce this feature in a later firmware update as thats all it needs.
The Wii can read dvd video discs, it just doesn't know what to do with them once they are inserted, a software update or addon will allow the wii to understand how to read them.