Onix was good in early LC, when Murkrow was available, as it was possibly the only real "counter" to the devastating SubKrow, but since then it's kinda fallen out of use. Also, without recovery, it really can't take hits all that reliably over time.
Also, for what it's worth, X-Act's stats thread shows that Pokemon with high Defense compared to the opponent's Attack fare better in OU than they do in LC., which is a significant part of the equation. Onix's mediocre HP hurts it, as well as the also-mediocre base 45 Attack. It's largely outclassed by Rhyhorn (80/85/95 physical stats are far more solid than 30/45/160 stats) and Geodude (40/80/100, but Sucker Punch access makes up for low speed).