Could be many reasons...
1) they know how dedicated the fanbase of From Software Soulsbornes are so they want to use a new one to push PS5 sales. At this point, it's not impossible that a new one would be ready within the first year or so of PS5's launch
2) there will NEVER be a Bloodborne 2. Sony will, however, continue giving From Software free reigns to develop new IPs (perhaps Soulsborne-like, perhaps not). New IPs take time, especially if it's really different from the Soulsborne formula.
3) it sold well but not well enough for Sony to fast track a sequel. It'll come when all the pieces line up or the higher-ups at Sony feel like it
4) Maybe Miyazaki is busy with another game so Sony is willing to wait until he's available to direct another game for them, since he's viewed as the mastermind behind From Software's most successful games.
5) It could be in development as we speak and will be one of the last hurraahs of PS4 (with a possibility of cross-gen) from Sony, to be announced at E3 2019 for a late fall or early 2020 release
6) For one reason or another, Sony simply won't work with From Software again (either because From Software is busy working with everyone else or because Sony has lost interest in the collaboration for the time being for whatever reason)