Comparing just to the Constitution of the Netherlands, that of Belgium was for a Unitary State with no substantial body of Common Law and tradition, while that of The Netherlands was for a Federal State, with a substantial body of Common Law and Tradition. Further part of the motive for the separation of Belgium from Netherlands in 1831 had been a feeling that insufficient Freedom of Religion was provided for in the Dutch Constitution.
Further, the 1849 decision by Denmark to base its constitution on those of Belgium and Norway reinforced the notion that it was a model constitution well suited for adaption elsewhere.
Of course, there is no written Constitution for either England proper or the United Kingdom, so using it as a basis for a written constitution is substantially more work.
The United States has a written Constitution, but is both a Federal State and a Republic rather than a constitutional monarchy.
The Empire of Germany was again a Federal State rather than a Unitary State; but was not yet a fully developed Constitutional Monarchy; the Kaiser still retained considerable powers that were presumably deemed undesirable in the 1906 Iranian Constitution.
In summary, all constitutions are different, meeting varying goals and objectives of the constitutional congress. It's less about which one is best, in an absolute sense, and more about which one is best for us.
So, inferring from the choice made, the Constitutional Revolutionaries of 1906 desired a Monarchical Unitary State, with minimal reliance on Common Law and Tradition, minimal jurisdiction of a Constitutional Court, strictly limited Royal powers and authorities, and substantive Freedom of Religion for the Sunni, Kurdish, and Zoroastrian minorities.