Sadly, it happens. When my husband was diagnosed with stomach cancer back in 2010 - he was incredibly fit - an award winning triathlete! I was AMAZED when one friend in particular never made contact after I'd advised him of his diagnosis! I don't know if he thought it was 'catchy'? We had supported him when his son was killed in a cycling crash, so it was even more weird. We caught up with him last year when he visited from NZ ..... so that was good.
I've been lucky since I was diagnosed in Jan - most of my friends & fellow Uke Players & yak fishos have been incredibly supportive of me. Stunned - but supportive.
Our life is made up of friends who come in & out of our lives - some for a short time, some for a long time - some for ALL the time!
Try not to worry too much about it - it is THEIR problem, not yours! ;)