Couple of things came to mind for me:

– abusive tweeters: we all know that most of us use social media responsibly but there are users out there (claiming to be football fans) who use social media as a vehicle for unnecessary abuse or vitriol aimed at clubs and players. These may account for a significant % of the ignored tweets. Whilst its important to highlight these users to bring accountability for their actions, I can see why those in control of the accounts would choose to ignore abusive/inflammatory/inappropriate tweets. The alternative is to report and/or block them.

– resource: as you’ve mentioned, I would wager a lot of the tweets that Chelsea ignored (ironically) were submitted during the time period before/during/after the greatest moment in the club’s history – the UCL final. Same for City. I’ve seen the stats for the UCL final and I just wonder if those working on the club account in the lead-up/during/post final simply couldn’t respond to the sheer volume of tweets coming their way? That doesn’t excuse it and I agree with your point at the end of the article – want to be a business? Start acting as one and resource these communications.

Cheers

Stuart