First of all, academics, just like everyone else, have a life. Something personal may have happened, which made it impossible for them to attend the meeting and reply to you. They may be sick and bedridden.
There certainly is a culture in Academia of bringing work at home (or rather never really stopping), which can expands even when people are sick or have personal issues, but that's not something to ignore.
Alternatively, academics are also very busy. They receive a lot of emails, and yours may have been missed. Some people are not very good at organising their schedule, emails, ... and will make more mistakes of this type, but it can happen to everyone.
While for you, that was the only event of the day/week, they may had 4 other meetings that day, and 3 other interviews during the week.
And plenty of other things could have happen. We can't say, and we can't properly answer your question, because only the PhD supervisor themselves can. They may send you an email in a few days apologising.
Now, about academia, as I said people receive a lot of emails, and you sadly would not be the first to stop receiving answer from prospective PhD supervisor or other academics. It happens all the time. Some academics will try to answer everyone, but some don't bother, and some may just apologise later.
I once received an answer to my prospective email to a supervisor 5 months later when they cleaned their mailbox.
I'm aware you had contacts with them, but just as they may have a totally reasonable reason to not have been able to contact you such as being sick, they may have also decided to not continue and not answer to you.