The FA Cup was massive because the final was one of only a handful of matches shown live on TV, and the only one where you could watch two English league sides. We've now got more live games in one week than we had in a whole season before Sky came along so the cup has lost much of it's status. I only take an interest now if West Ham reach the sixth round, as for the final I'll watch if I've nothing better to do (Hammers appearances excepted). Other than that it is just an interruption to the league program.
Good point about live TV games, which I hadn't thought of.
For many years, the group of mates who I go to watch Everton with have an end of season day out in town on FA Cup final day. Several other mates who support Liverpool also join us as well as a City fan and a Villa fan - unless Everton happen to be in the final in which case we go to Wembley, but that hasn't happened since 2009. We dish out prizes for our prediction competition, which usually consist of large alcoholic drinks, have a meal and get very drunk, so cup final day, for us at least, is still a big day, but I know that doesn't mean it is important to others.
I don't think the football powers that be (and TV companies again) have helped either by making it a 5 o'clock kick off in recent years. I don't know why they did that, but I think it has been another decision that makes the game that used to be an annual national showpiece, traditionally kicking off at 3.00 pm, appear a bit less important again.....plus, whereas on our end of season day out we used to be able to eat around 6.00, we now have to wait until about 8.00, it is just so inconsiderate