Because they're better fit together than Ai and Conan are. :grin:
But in all seriousness... AiCon wouldn't work out.
Haibara and Conan would not be good together in my eyes. Just.... no. They don't share the same morals, first of all. Haibara, if a man walked into a bank in front of her, robbed it and killed somebody on the way out, would simply call the cops and play it safe. CONAN, however, would run RIGHT AFTER that penniless (or now that he robbed a bank very rich) murderer and knock him down. You see, Ayumi would do the same thing as Conan and follow him while Haibara would no. This makes Ayumi more compatible moral-wise than Haibara to him. Ayumi and Conan would raise a child to believe they should chase after that criminal. Haibara and Conan would raise that poor child to be confused and misguided. XD They wouldn't know WHAT to do or WHO to listen to if a somebody robbed a bank and killed someone in front of them. Listen to Mommy or listen to Daddy? Hmmm....? That would tear the poor little thing apart! (Unless, of course, he/she already HAS a favorite which would not be a good sign...). Likewise, Mitsuhiko would be on Haibara's side and their kid would be raised to lay back, call the cops, and hope for the best. This makes Mitsuhiko more compatible moral-wise for Haibara than Conan.
Now to the second issue with AiCon- Conan would be reminded of being poisoned, shot, knocked out, the fear he felt and all the lies he had to give and all that he was forced to leave behind whenever he's with her. I don't think he'll ever be able to see her as a romantic partner when she, on some level, RUINED HIS LIFE. Ayumi, at least, resembles Ran in a very strong way such as her popularity, her morals, her looks, her fears, and her reactions. He would probably get more of a nostalgic feeling and not feel like he lost as much. He would still be reminded of it every once in a while, but Ayumi is not a PRIMARY SOURCE of his pain like Haibara is. Mitsuhiko, also, has NOTHING against Haibara and wouldn't have any problem seeing her as a romantic partner.
Now, to the third point. Haibara ALSO has very little in common with Conan. Besides soccer and being shrunk and living in fear out of the same organization, they have NOTHING. Haibara, while once temporarily interested in detective work, ultimately chose science. Conan doesn't CARE for science although it is needed to be a detective. If he didn't need it to do his job he'd probably have no interest in it. Once they both got home neither would be all that interested in their work. MITSUHIKO, though, is a very smart boy and knows a LOT for his age. He seems to know plenty of scientific facts and often shows interest in things that would lead us to believe he'll be a scientist or something related in the future. Ayumi as well is interested in detective work so that gives you a clu as to future jobs she may take up. For all we could know Conan'll be a private detective and she'll be in Sato's spot. XD
She has shown no real interest in him and he obviously has had no real interest in her so far. Haibara has been hinted to have feelings for Mitsuhiko.
(I have already posted this ONCE in the AyuCon thread... now here. :grin: )