As Steelcan’s stickied comment notes, it’s hard to answer this question for a few reasons - first, the contemporary medieval sources don’t really tell us; two, as far as I can tell, modern historians have never really bothered to speculate about this; and three, even if they did speculate about it, we would still have no actual idea, because of the first reason.

For the first point, the only Greek account of the Fourth Crusade doesn’t mention this at all. The last time he mentions Agnes/Anna is when Andronikos I killed her first husband and married her in 1183. He mentions Theodore Branas frequently, including his service in the Latin Empire, but never mentions his relationship with Agnes, so either he didn’t know, or he didn’t think it was important.

The two main French accounts of the crusade are Geoffrey of Villehardouin and Robert of Clari. Villehardouin simply says they were already married. Clari has a somewhat longer account of the crusader barons meeting with Agnes. She refused to talk to them, and then eventually had to talk through an interpreter because she had forgotten how (or refused) to speak French. Clari notes that they:

“…asked about the sister of the king of France who was called the French empress, if she were still living. And they said yes, and that she was married; that a high man of the city, Branas [Vernas] was his name, had married her, and she was living in a palace near there.” (Robert of Clari, pg. 79)

In French (and Latin) Branas was usually known as “Vernas” or (as if his name was a title, “Livernas” or “li Vernas”). The only source that goes into any detail is the Latin chronicle of Alberic of Trois-Fontaines. Trois-Fontaines is in Champagne and Alberic is generally pretty well-informed about things that happen in northeastern France and Flanders, which is where most of the Fourth Crusaders came from, although of course he himself was not present on the crusade so he’s not an eyewitness. According to him,

“…the man called Livernas maintained, in place of a wife, the sister of the king of France, the empress, who was spending her bride's gift and whom Alexius, the son of Manuel should have married. He had not married her in solemn nuptials because she had, in the manner of her clan, squandered her first bride's gift. Indeed, we shall speak in its [proper] place of how he later legitimately wedded her and about the daughter whom he begot from her.” (Alberic of Trois-Fontaines, in Andrea, Contemporary Sources, pg. 292)

There’s a sort of anti-French theme here, since Alberic seems to think they couldn’t get married because she already squandered her dowry, as the French always do. Those wacky French! He never actually does “speak in its proper place” about their marriage, all he says is:

“Prince Livernas was brought there [to Mytilene] so that he could join in legitimate marriage with the empress, the sister of the king of France, with whom he had consorted up to this time without legal nuptials.” (Alberic of Trois-Fontaines, pg. 307)

Their daughter later married a crusader baron, Narjot de Toucy, but we don’t even know her name.

As for modern historians, the issue is apparently not important or simply unsolvable and not worth worrying about. Phillips accepts that they were already married (pg. 191) Van Tricht also simply mentions that Agnes was married to Theodore but does not mention when it happened. (pg. 176-177)

Personally, my guess would be, if Alberic is correct, then maybe they had to get married as a condition of Theodore serving the Latin Empire. Maybe the new Latin rulers didn’t recognize their marriage in the Greek church and wanted them to get married by a Latin cleric (apparently, the new archbishop of Mytilene). Or maybe they never got married before because it seemed inappropriate for a lesser noble to marry the empress and widow of two emperors. But like I said at the beginning, we just have no idea.

Sources:

Alfred J. Andrea, ed., Contemporary Sources for the Fourth Crusade (Brill, 2000)

Robert of Clari, The Conquest of Constantinople, trans. Edgar Holmes McNeal (Columbia University Press, 1936, repr. 2005)

Joinville and Villehardouin, Chronicles of the Crusades, trans. Caroline Smith (Penguin, 2009)

O City of Byzantium: Annals of Niketas Choniataes, trans. Harry J Magoulias (Wayne State University Press, 1984)

Jonathan Phillips, The Fourth Crusade and the Sack of Constantinople (Pimlico, 2005)

Filip Van Tricht, The Latin Renovatio of Byzantium: The Empire of Constantinople, 1204–1228 (Brill, 2011)

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!