Because developing content for a stable platform is usually good practice and being a new user, he didn't know that 1.0 was such an old mess. I started with 1.0 as well, although it wasn't as old at the time, I had no idea of its age so that wouldn't have influenced my decision. Unfortunately, our developers have an aversion for decimal numbers and never updated to 1.1, 1.2, etc.
Developing for code that evolves daily is bad practice because you spend more time debugging new interactions (which may themselves change tomorrow) than new content. Suggesting that users use code that evolves daily is also bad. The way Yankes handles OXCE actually enables a proper development cycle, with regular but not overly frequent updates. OXCE+ w/ XPiratez or XComFiles is also pretty good.
With the main branch, we follow the nightly because there's no alternative except the obsolete 1.0 and modders are too headstrong to all agree on a given nightly to become the "unofficial 1.5 for which all mods are designed" (we tried) and too hungry for new features. That we do it doesn't mean it's good or that others should blindly follow.
I think the best is for modders to develop for OXCE or OXCE+. More options, more interactions with the coders, and a much more sane development cycle.