Fitness, training, proper form and the usual other stuff all apply as noted to better climbing out of the saddle. And that may be all there is to it for you as it certainly is for most others. But...certain people regardless of fitness etc are not built to climb standing. Probably the most common stereo-type category wise come from the tall and heavy. (For a cyclist.) We've all seen people like Contador, Pantani and Armstrong ride remarkably up those TdF climbs standing for long periods. (Especially on the attack.) On the other hand how often did we see Ullrich stand? Or certain other elite world class riders past and present?

Now I'm neither elite or world class. Just old and tall. I am however very fit and not overweight but at 6'5" and 215 pounds I'm not light. I've learned as an individual cyclist that I simply cannot climb standing out of the saddle except for very short distances/duration on climbs.(Yes, I've trained standing and continue to train but I only can stand a bit further/longer as a result.) If you are older as in being a senior climbing tends to jump you HR up very quickly toward it upper limits if you also fit those are areas that I named limiting standing.

Hopefully you are of the age and body type that will allow you to have the potential for much greater standing performance in you and it's just a matter of training and that other stuff to dramatically improve.