When many of the less intelligent animals are struggling to memorize Animalism's seven commandments, Snowball reduces them down to a single statement: "four legs good, two legs bad." This, he says, expresses the essentials of those core principles, and would suffice to prevent an animal from falling back under human domination.
One of the reasons that this makes for such effective propaganda is its simplicity. In this, it provides a marked contrast with the more complex and nuanced seven commandments, which, themselves, could be understood as a simplification of Major's original vision. This simplicity allows it to be easily memorized by any animal on the farm, a theme expressed with the sheep, who become fanatical about reciting it, while also allowing for ease of understanding.
In addition to its simplicity, also note that this maxim is based in bifurcation. By dividing the world into humans ("two legs") and animals ("four legs") it instills in all the various animals a shared common identity, defining them collectively in opposition to their human antagonists. In this respect, you can see parallels to real world propaganda, which, likewise, often tends to appeal to a shared sense of common identity, for example, national or class identity, while often invoking an antagonistic other that needs to be opposed. These same techniques are present in Snowballl's maxim.