Why would he give a fuck about airi when he got a great position by marrying Kylie? He got the status of nobility, he is respected by everyone as kylie's husband etc. What did he have while he was with Airi? He was living a very poor life before. No one respected him before.
Airi is the slave so she don't have any choice. MC is drowning in power so it's natural that he forgot the one for whom he started everything.
Now his no. 1 girl is Kylie, both officially and unofficially. Unofficially it should have been Airi. Her being a slave shouldn't have changed MC's love and position of her.
Airi is considered as equal to Kylie but is she really equal? Nope. Both Airi and MC are aware that MC loves Kylie the most, Airi doesn't sleep on MC's bed, she's still not pregnant whereas MC already has 2 children. Kylie's slave sleeps with MC on his bed but Airi doesn't.
Airi was the main girl but now she's nowhere near top. Airi deserved better.