The issue is not that a third party can read contents, it is that a participant in the protocol (which could encrypt/decrypt) can do malicious things, so encryption wouldn't do anything.
Newer proposals include signatures and other steps to solve this, but as the article discusses uptake is slow and it makes operations more complicated. The weakness of BGP against hijacking also means that it's quite simple and very flexible, which operators like.
(I misunderstood your question and thought you referred to BGP protocol data, which is why I mostly answered about BGP, but...)
The attacker could have just thrown the data away, totally interrupting all these communications. Even if encrypted, traffic patterns could be interesting and harder to obtain otherwise.
Newer proposals include signatures and other steps to solve this, but as the article discusses uptake is slow and it makes operations more complicated. The weakness of BGP against hijacking also means that it's quite simple and very flexible, which operators like.