if Chiyo and Ookuma were in such an intimate relationship in Middle School to the extent that they would probably have become boyfriend and girlfriend had not Ookuma become a complete dick after hearing rumors about Chiyo and then never spoken to her again, why include him as an active participant in the story to begin with?
Fine, Chiyo suffered "love trauma" because of the way Ookuma treated her. That was all explained in chapter 14. Okay, now we know why she had a hard time dealing with Igarashi when they got reunited. Is there a reason to further include Ookuma as an active participant in the story? Nope.
Unless, of course, your long-term plan is to make Ookuma an actual love rival to Igarashi. So, you make Ookuma Igarashi's senpai at work (oh, what a coincidence). You have Ookuma and Chiyo cross paths a couple of times, and then when they are "introduced" by Igarashi, they both deceive him into believing that they had never met each other before. Chiyo has a brief (and I do mean brief) hint of conciousness that she ought to come clean and tell her boyfriend "that stuff about Ookuma" being the one who caused her "love trauma," but that all lasted just 2 (count them, 2) panels and it hasn't crossed her mind since chapter 55.
Ookuma clearly still has a thing for Chiyo, as demonstrated multiple times, and she has never forgotten him, either. We will never know if Ookuma was just going to apologize to Chiyo at the festival, or if he was, as Igarashi thought, actually going to confess to her. Chiyo has even used her memory of time spent with Ookuma to try to make Igarashi jealous (chapter 122).
We are constantly being reminded of Chiyo and Ookuma's past by his continued active presence in the story.
So, why is it a big deal that Chiyo is still keeping a secret with Ookuma instead of being open and honest with her boyfriend about it? For the same and primary reason that Igarashi not telling Chiyo the whole truth about Subaru is wrong: you should not lie to or keep secrets from your significant other if you want your relationship to last for a long time.
How does that past "relationship" have a bearing on Chiyo and Igarashi's current relationship? In more ways than one. As a result of having suffered that "love trauma," Chiyo has become a top-level tsundere at whose irrational outburst Igarashi suffers. It is also the reason why she does not trust Igarashi to be alone with other women (though it's fine for her to be alone with other men). And it is also likely a contributing factor to Chiyo's readiness to be deceitful.
I have long thought that, if Chiyo were to actually apologise to Igarashi about decieving him, confess that she and Ookuma had really known each other in Middle School, and that she had had a crush on him, she could become a much better person by lifting the burden of her past off her shoulders. Unfortunately, after more than a hundred chapters, it's doubtful that will ever happen.