Why did the Shadow Hearts series end?
No cancellation announcement ever came, because there was nothing so clean. The series belonged to Aruze, a pachinko manufacturer (today Universal Entertainment), which had acquired developer Sacnoth along with SNK and renamed it Nautilus. Inside Aruze, game development was always the smaller business: Nautilus staff split their time between Shadow Hearts and supporting pachinko production, and when Aruze reorganised in 2007, the studio was renamed Aruze Global Trading and exited game development altogether, then merged out of existence on 1 February 2009. Plans to continue the series simply had no studio left to execute them.
The economics did the rest. Each game had cost more than the last: Covenant, the beloved one, sold around 240,000 copies in Japan and North America, a success for a cult RPG and a rounding error beside the parent company’s gambling revenues. From the New World, made cheaply and quickly to widen the audience, debuted to low sales. The rights remain with Universal Entertainment; Japan got a Shadow Hearts pachislot machine in 2008, and nothing since: no sequel, no remaster, no re-release, on any platform, ever.