According to Nickandmore.com (which covers ratings and audience numbers for children's cable channels), DD went from a series premiere of 1.8M viewers to a low of 571K for a first run a few months later. Second season episodes looked to hover around 700K.
Younger viewers are turning to streaming options more and more, and I just have that feeling kids are not as interested in watching a show like DD online compared to, say, SpongeBob or Loud House. (Also, could one argue that DD 2019's target audience ended up being the parents who remember the 80s version. There are so many options on cable, streaming, YouTube, etc for family friendly stunt based shows.)
You can also rerun episodes of SpongeBob, Paw Patrol, etc to the gills and maintain strong ratings. DD can't do that, and Nick is heavily invested in finding shows with strong repeat potential.
JD