So, as an old soul who went through this process many years ago, here's my take...
I got offers from multiple PhD programs the week of or after interviews (in my case, some time in February). I waited on my 1st choice up until April 15th (for which I was told I was an "alternate") That day (4/15), I still hadn't heard from my program of choice. I called them repeatedly throughout the day and wasn't told until 4:45pm that they were giving the spot to someone else. I had to hustle to get my letter of acceptance into my next favorite program - which, coincidentally, had been contacting me regularly since my interview, asking about my intentions. All this to say, when a program REALLY wants you, you know it (and they make it very clear.) When they are waiting on someone else to accept or reject their offer, it often takes until the last minute. It doesn't mean they don't want you, but it does mean they want someone else more. As a result, you should be ready with an acceptance to your 2nd choice school, or else they might give away the spot...