Please explain this to me!!!

So I have been dating my boyfriend since high school. Our relationship is as close to perfect as it can be, yes we argue over stupid things but we talk it out and trust each other. We are long distance but like I said never had any issues. He’s in a fraternity and every year they have a formal in April. I went the past two years being long distance I had to fly up and plan in advance. The first year he didn’t technically invite me since I kinda was like can I go and he’s said of course. Last year he didn’t tell me the date until the week before! I legit scrambled to find a drsss and for a flight. Since he didn’t tell me until last minute their were no hotel rooms left and we had to share with a friend of his, super weird situation . Both times I was like this is strange and all but let it go since like I said we have a good relationship. Well here I am now and I knows it in a month since a friend of his gf posted the invitation on Snapchat. I just don’t get why he never invites me in advance and show effort like it’s a couples event everyone has a date! Last year I called him out and was like were you gonna take someone else or something and he’s like no he just didn’t think about asking me. I just honestly don’t get it at all like how does that slip his mind. I thought also maybe the first year I drank and was embarrassing and barely drank anything last year. After he said how much fun he had. I see him next weekend I don’t want to beg for an invite but how do I mention it to him without being too obvious?