I think they saw how SpaceX was using Starlink as launch lever to provide SpaceX a baseline of regular launches at bare-minimum cost. As RocketLab starts to scale up, being able guarantee a minimum number of launches is a significant hedge against the dips in the global satellite market.

Also, RocketLab builds their own sats and can add the Iridium constellation replacements to their order book. It's a win-win. A smart move by Peter Beck and his team.