The simplicity is a prime thing. Even though I'm a good programmer, I love being able to open RPG Maker and just start building a game world without having to write a line of code. I can focus on the story, not get completely, utterly lost in "How do I pass events around?"
The incredible ease of debugging while the game is running, expanded even more dramatically with some debug scripts. It's very easy to see exactly what the game is doing in terms of switches/variables/etc, which really helps in fixing problems.
The built-in resources are also an excellent starting point, giving everything a beginning game maker needs to make a complete game. So I don't need to hunt down game themes, sound effects, battlers, tilesets and assemble them into a coherent package --- RPG Maker comes with one right out of the box.
The inexpensive custom resources available at the store. For only $30 (or less) per pack, I can get graphics resources to tell a huge variety of stories, music and sound effects galore. Basically for the cost of a night out, I get whole new toolkits with which I can build completely different worlds.
The extensibility --- there are literally thousands of scripts available to do everything from complete battle system rewrites to item crafting, weather, game time. And if I want, I can roll my own, for free using Ruby (which itself is freely available). Most of these scripts are "drop it in and go" as well. And all of this builds on the entire existing framework so I don't need to re-implement Events, for example. So I'm not just limited to what RPG Maker can do out of the box. Some people have written entire mini-games for their game, using custom scripts.
Also, having a very active, supportive user community is a HUGE thing. I'm sure many of us would have been stopped dead in our tracks at some point or other, including me, if we didn't have literally thousands of people in an active, quite civil forum who were willing to help each other in a wide variety of ways --- from "Here's how to make events do this" to "Here's how to create autotiles" or "Here's a custom character portrait"
A fair amount of otherwise impressive open source tools have died on the vine because they couldn't grow a community large enough to be self-supporting, and RPG Maker doesn't have that problem, which is probably a good part of the reason why it's still around after over, what, 20 years?