Because in engineering, the question is not "why not", but "why". Blockchains are not the default answer. What positive virtue is blockchain going to bring to your IM system that is not better attained by a non-blockchain-based solution?
If you can answer that, great. I'm not saying this has no answer. I'm just saying it's the real question.
If you are going to say the word "federated", then from what I can see, trying to do "blockchain" at that point isn't all that helpful and likely to be worse and harder than just trying to develop a federated system, a sufficient challenge on its own that doesn't strike me as greatly helped by "blockchains". XMPP already does federated IM, since long before the current fad.
I match any vague handwaves about identity or security with vague handwaves at existing SSL certificate authorities, and an observation that if you're going to complain about the security of such things I get to apply an equal amount of effort towards breaking your blockchain solution, e.g., if you're going to complain about nationstates getting root certificates they shouldn't, I get to point out your tiny little blockchain for IMs is equally vulnerable, if not more so.
Though I will concede your point slightly; if XMPP needs some more Hype Juice to win, I can't deny "but on a blockchain!" could help there. Kinda not being sarcastic here.
To the declining popularity of Jabber, to put it short.
As an additional benefit, blockchain is indeed more secure to run than an ejabberd instance.
I understand your irony with "Hype Juice" etc. But in the end, good software is that which people use, right?