but instead rotate through in order to avoid arousing suspicion

Also to use fewer resources! Compared to the SIM cards, the radio/modems are expensive. It's cheaper to reuse one radio/modem cycling through different SIM cards for just long enough to receive pending SMSes. But not too many, or you increase latency. It's probably more suspicious to have the same modems cycling through SIM cards than to have all of them always connected.