But they are different kinds of conversation.
Here's something that happens to me a lot. I get an IM asking me a question, in order to answer it I need a bit more information so I ask a question. I wait. Just at the point where I think there's no reply coming and I can re-focus on whatever I was doing before I get an answer. I reply, adding a further request for information. Repeat a couple of times until I finally say "Send me an email".
If that was a phone conversation the person would generally answer immediately or at least I would know what was happening. Silence on IM might mean they're thinking about my question, or it might mean they've been distracted by an email.
The problem with IM for me is that it's somewhere between email and phone in terms of being "live". It can come across as passive agressive because it demands attention right now but the person at the other end can more easily split their attention across other things. A phonecall would require both my and the other person's attention equally. An email requires my attention but I can prioritise when I reply.