The reasons are related to anatomy of spinal column. In adults the spinal cord ends at lower level of second lumbar vertebra. The sheaths/coverings of spinal cord extend all the way down to cocyx. This creates a hollow tube filled only with cerebrospinal fluid from second lumbar vertebra down to end of spinal column. Most of the epidural anesthesia techniques and all of spinal anesthesia techniques use this space because there is no risk of damage to spinal cord. In epidural anesthesia, the catheter can only be advanced a few vertebra up and down from point of insertion. The regions on which surgery can be performed depend on reach of epidural catheter. So, with an epidural catheter in lumbar region, only lower limbs and lower abdomen can be anesthetized. With epidural cathetr in lower thoracic vertebra , approx. lower 2/3 of abdomen can be anesthetized. For chest surgeries you will need epidural catheter in mid thoracic vertebras. And so on. But the problem with thoracic epidural is risk of puncture of dural sheath and damage to spinal cord which will result in permanent paralysis of that area. Epidural anesthesia can be used for upper abdominal and chest surgeries but the risk will be more. Moreover, for extensive surgeries a lot more is needed just simple loss of sensation. for example, more granular control of heart rate, blood pressure, oxygenation etc ; all these manoeuvres are a lot easier in general anesthesia. So for lower limbs to lower abdomen , spinal and epidural anesthesia are fine, even better compared to general anesthesia (especially in obstetrics). For the rest, depends on specific circumstances.

Lots of surgeries are done this way- you can do regional blocks for limbs for example, and in high risk patients for knee replacements this is common.

The main reason why it isn't used more often is 2-fold: there is a risk from spinal or epidural anaesthesia of infection or paralysis since it is an invasive procedure; and secondly an awake patient is a risk unto theirselves: they could move, they may talk (?annoying to the doctors)

If you were undergoing a procedure you could very likely elect to have a regional block if you preferred. Just make your preferences known to the doctors