[wickedshimmy]

They can annihilate -- but annihilation channels are OZI-suppressed because of gluon effects in e.g. charmonium (the J/psi, a famously narrow resonance, which was one of the final states of the decay that produced this signal). The same mechanism is likely in play here as well (it is an artifact of QCD) -- the idea is that hadronization can be energetically favorable to annihilation: since the gluons carry a strong charge (color), they participate in the interaction and can nucleate a light q/q-bar pair rather than requiring an additional (virtual) gluon to couple to the annihilation vertex.

(Also it does happen with non-quark particles as well! Positronium for example, is an electron-positron bound states -- we just see more stable hadron particles like this because QCD has Weird Maths).