Why is DARPA interested in secure messaging platform using blockchain technology?

I see your question as having two parts:

Why blockchain?

According to ZDNet's summary of the request, the preference for a decentralised blockchain solution is rooted in an interest in moving away from legacy centralised architectures, which DARPA describes as "expensive, inefficient, brittle and subject to cyber attack" at the given link.

Why not an existing solution?

From my view, DARPA's request for proposal is justified before even assessing the advantages of using a blockchain solution in particular.

After reading the summary by ZDNet, two reasons come to mind:

• Specialised requirements: The alternatives you list are for text (and basic attachments) only. By comparison, the linked article refers to planned use for Military Interdepartmental Purchase Requests (MIPR), described here in Oracle documentation. Verging towards a procurement solution, this requirement alone is beyond the capability of what you have listed. Also, without significant redesign, those solutions would not be capable of catering to military user groups and access levels.
• Dependency on third parties: More generally, it makes sense for any defense organisation to own the lifecycle of such a critical platform. Hypothetically, if the listed solutions did meet DARPA's requirements, to rely on an existing third-party solution would introduce unnecessary risk (e.g. non-DoD users on the platform, limited or no authority over design decisions, existing solution architecture being unlikely to meet defense standards). Although it is unfeasible to reinvent every technical solution in-house, in this example, little would be saved by relying on the listed alternatives.