It's a relatively small market of demand, with supply exceeding that demand. This tends to drive down earnings levels. I suspect (but don't know) that many folks qualified as 'town planners' actually do very different things in their careers. Of all the chartered professions - engineering, accountants, lawyers etc. - civil engineering and architects tend to get the rough end of the stick, earnings-wise.
Higher earnings are achieved in private practice - planning consultants, environmental consultants and the like.