Lack of patience coupled with lack of confidence.
Conversion work (to me at least) is easy and takes virtually no time. You cut, you glue, you;re done. Sure, you can spend time fabricating parts and there's a lot of time spend figuring out what pieces go where but generally speaking, it;s instant gratification and you can generally repair anything you mess up without too awful much work.
Painting, however, takes FOREVER if you;re doing it right. There's sooooooo much technique and skill and talent involved.
Even a run of the mill paint job requires several repetitions of the 'paint/dry/paint/dry' song and dance. Meanwhile if you want a GOOD paint job there's multiple layers, thinning of paints, ink washes, etc.
Not only do I have no interest in literally watching paint dry, I don't want to ruin my pains-taking conversion work with a sloppy less-than-perfect paintjob.
So all my beautiful conversions remain unpainted and will probably stay that way til some enterprising pro painter decides to paint everything I make for free as some kind of crazy PR