There's not really any reason to wait 6 months. 6 months is the entire cycle, from microfilarid in the original infected host to a reproducing, adult heartworm. It has to go through a mosquito somewhere in between there.
45 days is about what you need, because that's the time period between when the L3 enters the host, molts to an L4 and when the L4 stage is still susceptible to treatment, give or take a few days.
We just say monthly because it's easier for clients to remember, and gives a lot of breathing room for prevention of what's otherwise an expensive condition to treat.