I'm not sure if there are any really clear advantages, but, the reasons I like it:
-It's got a code editor, sprite editor, map editor, sfx editor and music editor all built in. It feels "cozy." Kinda like how QBasic felt when I was a kid--you rarely needed to leave the IDE (depending on how complex you were getting with your projects)
-The palette is fixed to 16 colors. I'm not much of an artist, but with this few colors I have been able to pull off acceptable graphics without a whole lot of effort, which certainly helps with prototyping. I'm not that good at selecting colors for "shadow pairing" and so on often used in cell shading, but that has already been done for me with this palette.
-It has a feature called "splore," where you can browse hundreds of carts uploaded by other users, and check out the code immediately. This also feels like QBasic, not because it was built in, but because I recall downloading many qbasic programs offered for free on qbasic websites and reading the code---and having zero effort required to build and run the code.
-It "feels" like an old computer due to the constraints and that much of the data is a custom bit-packed format. It even exposes ram that you can call PEEK and POKE on, which further feels like an old computer.
-It has an api for drawing sprites, tiles, and a variety of primitives.
-Lua provides coroutines...which I've come to really like using for entity state machines (I'm effectively using coroutines in my current NES game, by saving the PC and entity's state when an entity is finished updating the current frame).
I guess the clearest advantage for me is, if I used a framework, I'd have to decide how to create a full screen, low-res graphics viewport...when I had to do this with LibGDX for ggvm, it wasn't that easy to figure out. Pico-8 just has that all done for you, which is nice for nincompoops like me!