I don't think it's a matter of people not knowing about it. For embarkation lunch, where we eat depends on who we are traveling with, what the weather is like and what time we've boarded. We typically eat at the quick service spots rather than Cabanas but if we are with my parents or the weather is iffy, we'll go to the MDR.
For the rest of the cruise, we go to Cabanas more often that the MDR for a few reasons - first we don't need to spend a lot of time on breakfast/lunch, more choices at Cabanas although my cruise breakfast is usually the same every day, and the most important is the hours of the MDR. They are usually limited. So on the days that we have more time to linger, we usually linger too long and then miss the hours the MDR is open so have to go to Cabanas anyway. We do try to do at least one lunch in the MDR per cruise and are usually successful. We do try to do at least one breakfast there but usually that isn't as successful and the one MDR breakfast turns out to be disembarkation breakfast!
One other thing that wasn't mentioned - sometimes if we are up during MDR breakfast hours and remember to head there rather than go to Cabanas on auto-pilot, if we see a line, we'll keep moving and head to Cabanas anyway. When it is crowded, they will seat you with others during breakfast and lunch. It's not usually a problem and I do like chatting with others on a cruise. But it is too much for me at breakfast.