Course syllabus have to cover a global need. DSMB is beneficial, but there's regions where the average diver will never need to deploy one.... other regions where they're virtually mandatory.
This is an important concept that some people have trouble understanding. In education circles, some people contrast
just in case learning with
just in time learning.
When I teach an OW class, one of the first things I do after students get in the pool is swim around on scuba, first on the surface and then under water in the shallow end of the pool. When they come up, I jokingly tell them that they have learned how to dive, thank you for coming, etc. Then I seriously tell them that nearly everything that follows in the class is really about how to respond
just in case something goes wrong.
When you teach things
just in case the diver needs it, you have to consider factors like the importance of being able to do it when needed and the likelihood that they will need it. If the factors do not indicate an immediate or especially important need, you are better off not teaching it, because the student will likely forget it by the time it is needed, and the time spent learning it detracts from the ability to learn the more important and more timely stuff. As an example, in deciding whether or not to teach out of air skills, you realize that the odds are the diver will never need it (I never have), but it will be both extremely important and unpredictable when it is needed, so it should be taught from the start.
With
just in time learning, you determine that the diver is unlikely to need the skill at that time, and when the diver needs it in the future, the diver should be able to predict it and be able to learn it
just in time to use it. As an example, the OW book gives only the briefest mention of tides. It does give some information, but it mostly tells the diver to get local knowledge if diving in an area where tides affect diving. I personally went nearly 900 dives before I was in a situation where tides made even the slightest difference in my dive planning--Puget Sound. I learned about them from local people before I did those dives. In contrast, someone who lives in Seattle and dives locally needs to know about tides from the very beginning.
PADI has made the decision that deploying a SMB is now important enough to make it a requirement for OW training. As for a DSMB, many divers will go hundreds of dives without ever seeing one deployed, so it makes sense to teach it only to people who will find themselves in a situation where it could be important to them.