Most popular =/= best nor most used (which seems implied by this statistic). Statistics is a tool that I can use to show you any result I want by carefully picking the categories.
Paid software does have better tools and is much faster (volumetric booleans, highpoly performance, etc..), so why didn't it win in popularity?
Know-how in paid-software communities is very guarded, most had to pay for tutoring/tutorials and not so many people are willing to share it for free (usually they gladly tell you when you ask them, but they won't post it online on their own). Between professionals if you are the only one that knows something in the world, you are worth all the money for a company. It is similar how companies guard their know how or they licence it to others for money.
You will find that Blender and SketchUp got all the winning points in the Forums/Youtube/Google category. Free software is available to everyone and they share. On the web, on social medial, everywhere. If this is the key metric no surprise it won.
Enough of this now.. So what tricks blender offers for 3D printing?
Many Import/Export format options - which gives you great flexibility. You can bring models from professional CAD or sculpt software and export as STL, OBJ, X3D, Collada or VRML97/2
Solidify modifier to make it hollow inside
3D Printing Toolbox Addon - this is a nice tool to analyze your mesh if it will print nicely
- You want to check minimal wall thickness
- If the mesh can exist in real world (is manifold)
- Check overhang geometry
- Check normals if all pointing out
Powerful modelling tools to fix any errors
Average/Good_Enough_If_You_Have_Time sculpting tools
Python scripting for automation and customization
Tutorials everywhere and great community
And it is free :) The only cost is your time.