So, as might be obvious, I'm currently DMing CoS, but I'm a bit stuck in how to proceed.

My group is currently escorting Ireena to Valaki and I'm planning to have them run into Strahd on the way. Thing is, they obviously can't defeat Strahd in battle, so he more or less has to leave on his own accord. What I can't figure out though would be why? Why would he just leave Ireena behind and disappear. It doesn't really make sense to me. But maybe I'm overlooking something?

Thanks in advance and a good day to all of you. 

Play the overconfident nature of Strahd. He toys with all he feels are beneath him(everyone). He makes a show of strength proving that the party can't protect Ireena, gloats and invites the group to join him at his abode. They will bring Ireena if they know what's good for them. He takes his leave.

Strahd likes to mess with people. Let him stay until the heart of sorrows is depleted and then he can laugh and transform to mist. They'll have dealt 50 points of damage to him without a scratch and they'll start thinking he's invincible. While they're doing that, he can try to charm a few of them into letting Ireena go. Remember, she's only one of his goals. He also wants to test adventurers to see if they would be worthy replacements.

Strahd’s an abuser, a narcissist, and a predator.  At this stage of the proceedings, he thinks he’s having a private conversation with Ireena.  Ireena, he thinks, is being foolish by not wanting to be turned into Tatyana again so they can start their love affair again (Tatyana never loved him either; she leapt to her death to escape him, or so Strahd tells the story).

This is the toxic freakshow who you see chasing a woman down the street, yelling “Baby!  Baby!  I’m just trying to talk!” and telling strangers to “mind your own business!  I’m talking to my girlfriend!.”  He might be curious about who the party are, but they’re not in the forefront of his mind right now.  He’ll leave because he remembers (in his version of the story) how “irrational” and “hysterical” Tatyana can be and when he can see that she’s really going to Vallaki, he’ll back away and rethink his tactics.

As most of everyone said. Strahd doesn't want to win right away. He likes to play with his food. He would much rather give them false hope and crush it later than instagib the whole party at minute one.

Yeah, like most people have stated... the good count likes to play, and Strahd is bored. The group does provide him with much needed excitement in his otherwise dull immortality. I recall he has very little love for the people in his own realm, seeing them as cattle at best, but the heroes... now they have fire, flare and makes comments like "you'll never get away with this you fiend!". In my book he wouldn't want that game to end before it even started. 

Quote from TimCurtin >>

Strahd’s an abuser, a narcissist, and a predator.  At this stage of the proceedings, he thinks he’s having a private conversation with Ireena.  Ireena, he thinks, is being foolish by not wanting to be turned into Tatyana again so they can start their love affair again (Tatyana never loved him either; she leapt to her death to escape him, or so Strahd tells the story).

This is the toxic freakshow who you see chasing a woman down the street, yelling “Baby!  Baby!  I’m just trying to talk!” and telling strangers to “mind your own business!  I’m talking to my girlfriend!.”  He might be curious about who the party are, but they’re not in the forefront of his mind right now.  He’ll leave because he remembers (in his version of the story) how “irrational” and “hysterical” Tatyana can be and when he can see that she’s really going to Vallaki, he’ll back away and rethink his tactics.

 I second this idea. I like the idea that Strahd will treat the party like they aren't there. If they try to intervene, he can take an action on his turn to try and "put them in their place." Perhaps Strahd gives a dinner invitation to Ireena and saying she is welcome to bring her group of "retainers" if she so desires (making sure to try and egg the party on by continuing to imply that they are not worth giving an invitation to). This can either be because he genuinley does not think the party matters or because he is trying to anger them, making it more likely they will come to dinner and charge blindly through his castle to their doom, delivering Ireena to his clutches.