Why can't I download Bridge for my PS 2019

Ostensably from the same place you obtain PS 2019. Adobe had a legal issue a year or two ago where they could only allow customers access to current and previous versions of software but only the "just" previous version. This obviously screwed users who were dependant upon plugins that did not work with current versions and there's no chance of that plugin being updated. If you contact Adobe they might help you, there have been exceptions.

However, you cannot mix and match Bridge with PS (or Illustrator) of different years and expect the same level of inter-application cooperations. You have to match versions (2020 with 2020, 2019 with 2019, etc.) 

So go into your ACC and look at the Bridge update and to the right you'll see 3 dots. Click on them to access the oldest one you can get.