Why isn't Sith sorcery used more in combat?

Many Sith Lords who clearly know how to do sorcery don't actually use it that much in combat. From an in-universe standpoint, why is that the case?

A few possible explanations:

• Some Sith are proportionally better at abilities like lightning than they are at sorcery.
  
• By a certain power level, more direct manifestations are proportionally more dominant.
  
• Sorcery is usually impractical to use in combat, and those who do use it have certain circumstances.
  
• The opponent has techniques to counter the sorcery.
  

Because it's not useful in combat against an opponent who is:

1. Comparably powerful
2. Of fit mental state
3. Knows roughly how to defend himself against it (willpower)

The Banite Sith turned combat into a science. The ancients are relics. Sidious would use sorcery if it was important; it's obvious why he doesn't.

Every major fight in TotJ devolved into a lightsaber duel despite one or both combatants knowing sorcery. Exar Kun, Zannah and maybe a select few others, through either sheer brilliance, favourable circumstances or a combination of such circumstances were able to bring sorcery to bear against a powerful, highly skilled opponent (Luke's circumstances have been covered at length by other people).

There's also the fact that sorcery seems to be something one is either born with a talent for or isn't. It makes sense that in ancient times, when sorcery was very commonplace, and with the blood purity thing the Sith were obsessed with, that there would be many bloodlines who were predisposed to a talent for it. And we know that Force abilities are heritable, even on a level beyond the biological (see Vitiatekorion's kids). It also makes sense that in later years, when sorcery was very uncommon, that there could have been certain individuals who had the repressed, for a lack of a better word, genes that could take to using sorcery if they were expressed. I suspect Zannah is one of these individuals and was able to express those genes by studying Sith Sorcery.

But again, when you are getting to late-Banite levels of power, knowledge and skill, sorcery is basically a cool party trick at best. They evolved over 1,000 years, and any knowledge of sorcery that was lost (and eventually regained and improved on by Sidious) was completely superfluous because the new innovations they came up with were hilariously superior in practical application.

So the short version is... evolution. Sorcery fell out of use and it's expression in genes/spirit genes became nearly unheard of because it failed to adapt to the environment and was replaced by something far superior.

Wyyrlok is a possible exception; it's possible that some Sith have a higher proportional talent for it relative to their overall power / skill in other areas. So Palpatine may be very adept at sorcery, but he isn't necessarily adept at it relative to his abilities in TK, TP, etc. That proportionality seems more likely to tilt in favor of conventional abilities both the later in era and higher in power one goes; it's kind of analogous to Bane thinking amulets were pointless.

Now that I think about it...I'm not sure if there are any examples of sorcery being successfully used in combat that aren't circumstantial, aside from I guess Wyyrlok vs. Andeddu. Zannah tapped into Ambria, Exar Kun channeled his powers through Kyp (and never really used combat sorcery while alive)...I mean, I'm probably missing some.

Didn't Zannah successfully use combat sorcery in the climax of Rule of Two, against the Jedi strike team? Pretty sure that's how she took out the one Jedi who was outmatching her in sabers. Other than that, yeah, I can think of very, very few examples where sorcery was actually effective in combat. I can actually think of more examples where it just seemed to distract the user and pretty much lead to their deaths, like Volfe Karkko vs. Vos

@“The Ellmist” Technically Kun Force the senate with sorcery , qualifying it as combat?

basically what ils said