Mostly because in my ADD, I either forget to add them to my calendar, or I lose the email under a ton of emails that arrive after it. I think digital invites are horrible because I used to look forward to seeing a fun festive invite in the mail that I could post on my bathroom mirror.
Everything seems so digitally integrated now. I know I can't live without my iCal. I'd forget a party if it wasn't written down.... oh wait... I did. FAIL!
Don't get me started on Facebook and Myspace invites. Oy. Facebook invites are 80% band shows (much like Myspace). The rest are jewelry/Mary Kay/I-don't-have-the-money-to-shop-at-your-party kind of stuff. I'm all for hanging out with friends, but send an email for cricket's sake.
Ok... I'm getting off my soapbox.
Recap: I'm more likely to lose a digital invitation than a paper one. Evite/Facebook Invites are impersonal and suck (also more likely to go to the spam email than my current address, btw)