Sometimes friendships do cool.
I have ex-school friends who I no longer speak to. Once I went away to University about 130 miles away (friend A went to a local University and friend B didn't bother - she worked for BA) our friendship became less and less. We didn't fall out but they made new friends at University and their line of work, I wasn't home that much and when we did meet up, we were all doing different things in life, had other friends who we'd become closer to (I became closer to Friend C from University) and 8 years after leaving for University, I saw my friends for the last time. We went from seeing each other several times a year to maybe twice and the 'icing on the cake' was when Friend B got married. My then partner (now DH for 7 years - together for 12 years) had been together a year and he wasn't invited. It was our anniversary on the day of the wedding and I had to make a choice. I don't think it went down too well and I never saw them again! I never fell out with them, maybe they did with me - who knows! But 11 years later, Friend B from what I have heard is still happily married with two children of her own, Friend A now lives in New Zealand with a young daughter and I'm happily married with 2 DCs of my own!
As it happens, Friend C and I also started to cool our friendship the year after I got married. She married about 2 years previous to me and once I was married, married life took hold, our lives went in separate paths etc. The last time I saw Friend C was 2 days before the birth of my DS nearly 6 years ago! We still speak via FB and the last time we spoke over the phone was probably 2 years ago.
Like I said, we all take different paths in life. Sometimes those friends take the same journey but more often than not, most take a different route and sometimes that route means physical friendships are severed.
Recently I have hooked up on FB with school friends from my primary days and also high school. We keep in touch via FB - comments on status updates, photo and video likes and FB messenger. I would never consider my friendships severed. There has been no bad feelings on my part and wish ALL my friends past and present the best for their futures.
Life is busy and the world is growing but time is still the same. We don't all have the time to maintain the many friendships that we build up over the years and sometimes many become neglected.
Look at this as your friendship is still there as she DOES still socialise with you but that she also has other friends that she wants to do things with too.
If it is bothering you so much tell her that as discussed so much, you were really looking forward to meeting her for a few days in NYC and that you're disappointed that you are not this time round. Ask her outright if it is a possibility in the future but if she is 'a conflict avoider' give her the option to politely bow out along the lines of; "I won't be offended if you didn't want to". Although you will be hurt, don't mess up a friendship over something that YOU'D be hurt over but she may be equally not as happy for doing something with you she didn't feel comfortable doing. Sometimes in our decisions, one of 'us' has to be the one who gets hurt; it's unavoidable.
Also any chance that your friend's trip was last minute and the other friends from another part of the country were able to get a last minute getaway? Maybe she thought it was too short notice for you.