My guess as to why they introduce a new DPS every expansion is just that DPS is the most approachable role. A lot of players don't want the responsibilities of tanking or healing (whatever is left of those responsibilities anyway), so I'd imagine there are a lot of people who wouldn't even try out a new job if it wasn't under the DPS role. I think adding a DPS job is just a fail-proof method to get the maximum possible amount of people hyped.

Like others say, there are a lot more DPS players than Supports, so if they added a new Tank and Healer, there probably wouldn't be as much total hype around the new jobs.