Sith failed to defeat/capture the Republic in history prior to Sidious due to infighting/betrayals. Jedi and Republic forces simply took advantage of these occurrences.
By nature, Sith philosophy promotes competition and quest for supreme power.
Sith still came close to wiping out the Jedi Order and conquering the Republic several times in history.
1. Great Sith War*
2. Jedi Civil War**
3. Sith Triumvirate***
4. Great galactic war****
*Sith failed due because Ludo Kressh turned on Naga Sadow.
**Sith failed because Malak turned on Revan.
***Sith failed due to infighting between the Triumvirate; Traya prepared Meetra to end the Triumvirate.
****Sith failed because Vitiate was betrayed. The seeds of this betrayal were sowed by Nyriss actually which led to Scourge plotting against Vitiate. Scourge's defection to Republic proved to be valuable for Jedi and Republic; he is the worst thing ever happen to the reconstituted Sith Empire.
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Sith then had to change tactics. They had to stop raising armies and attempt to conquer the galaxy and purge the Jedi in this fashion. They had to infiltrate the Republic from within and plant the seeds of its transformation. They had to wait for the Jedi Order to grow out of touch and loose their edge over ways of the Force. They had to fuel the corruption within the Republic and use this development to its advantage. Eventually a time came when Sith were in the position to put their grand plans in motion and Sidious fit the bill, thanks to preparation and guidance of Plagueis.
Its not like as if Sidious magically became the most successful Sith Lord in history. Lot of developments led to his rise and paved way for his success; the Sith that preceded him deserve credit as well.
People can learn a lot about these matters from following sources:-
- Star Wars: The Old Republic: Encyclopedia
- Star Wars: Darth Plagueis
- Star Wars: The Old Republic: Revan
- Star Wars: Darth Bane: Path of Destruction
- Star Wars: Darth Bane: Rule of Two
- Star Wars: Darth Bane: Dynasty of Evil
It is vital to consult these sources to learn about failures and successes of Sith and associated reasons.
To be honest, Vitiate's influence over galactic events and accomplishments are massively underestimated.
Vitiate;
1. Saved Sith from extinction in the aftermath of Great Sith War.
2. Reconstituted the ancient Sith Empire in regions unknown to enemies of Sith and shaped in to a highly disciplined civilization.
3. Granted ample time to the reconstituted Sith Empire to grow and become powerful enough to challenge and defeat any kind of superpower in the galaxy.
4. Worked on a secondary plan to eliminate all kinds of galactic threats with the power of the dark side, should his Empire fail.
5. Infiltrated the Jedi Order and Republic with his puppets to learn about their form and function prior to ordering the invasion.
But Vitiate's plans flopped due to same reasons; infighting/betrayals. It shall be noted that Vitiate went to extreme lengths to succeed in his plans (SWTORE offers excellent explanation of tactical brilliance and power of Vitiate) but The Force have its own will and their is no permanent for infighting/betrayals.
In-fact, Palpatine also fell due to betrayals; first from Vader and second from Luke.
So I don't see the reason to declare Sith preceding Sidious as failures just because they grew up in different circumstances then Sidious and made decisions accordingly. In-fact, it can be argued that Sidious is the luckiest Sith Lord in galactic history.