To answer the question why, I'm going to start with some background. If you are the kind of person that wants "just the facts", skip to the end. The next-to-the-last paragraph answers the question very succinctly.
When I began writing for Nevermet Press, there was some discussion about creating statistics and details for various game systems. Most of the writers played 4e, so that was the big focus. I wanted to help, but since I have never played 3e, 3.5e, or 4e, all I could do is generate stats for systems in less demand. I had wanted to do OpenD6 conversions because I love the system, but WEG's apparent implosion led me to look elsewhere.
When I returned to gaming about four years ago, I had heard of various retro-clones that used the Open Gaming License to create the feel of earlier versions. Since I have only played earlier editions of *any* rpg (Marvel, Shadowrun, D&D, Ars Magica, Hero, etc.), these neoclassical rulesets seemed like the perfect place to start. I downloaded several and eagerly read through the rules.
What followed was a tremendous sense of nostalgia. I read OSRIC and remembered all the games my high school group played until 3am. It was great! I also read many others, all with similar feelings. I went into my RPG library and dug out my B/X rules, BECMI rules, 1e rules and 2e rules. I skimmed through most of them before diving into my notebook of house rules. (Note: It's not this set of house rules, but I like the name.)
Finding the Great Tome of House Rules helped me remember the way my group played the game. We amalgamated rules from almost every version we had. We used 1e stat blocks for everything, we had THAC0, we had no limit to the number of attacks a fighter gained against 1HD or less creatures. We also had certain NPC races as their own character class (There is no good link for were-swans). We never found a spell we didn't like. We loved all character classes as well. We had guns and lasers. All in all, our rule system was best described as founded in 1e, but influenced by the simplicity of B/X & BECMI, using all the settings and kits of 2e. There is no good description for the skill system we used. Please don't ask me to try.
Against this background, I also discovered Microlite20. This was my first experience with anything like the 3e ruleset. Reading the short rules and various add-ons the community posted, I was hooked. M20 has proven to be something between a beer and pretzels rpg and a full-fledged ongoing campaign. I could use my scribbled campaign notes and world creations without too much revision. It allowed me to ditch almost 95% of my house rule notebook. In the anything goes add-ons on the site, it was simple to recreate my beloved Spelljammer and blast off into the ether. It also turned out to be a simple matter to setup some of my previous settings that included so many classes and kits.
I had attempted a Psionics add-on. I helped write a Conan add-on. I had notes for various magic systems strewn about two thumb drives and a few computers. Working through creating these add-ons had inadvertently allowed me to become somewhat familiar with d20.
The other reason that I enjoy M20 so much is the community. There seems to be at least one or two posts a month that start with "I was thinking about creating XYZ in M20." It is possible to make M20 into something very far removed from the world's most popular role-playing game. More than that, the idea of creating something new is enthusiastically embraced. Many folks, including myself, lurk around the formulas waiting for the next new thing. (Take a look at the new traits plug-in. Feats without the headache.) It's now a part of my Monday ritual.
All of this may be a fun trip down memory lane. It seems to get away from the main idea of this post. In the end, why Basic Fantasy for my fantasy rpg of choice? I want a retro-clone for which I can stat NMP material. I enjoy something that is based in d20. I enjoy that it feels like the games my high school group played. I like the breadth of situations covered in core rulebook. I enjoy the open community that actively embraces new ideas. Most importantly, I have fun doing it.
Next up: Through the Basic Fantasy core rulebook.