A script that stays in use changes with the languages written in it. Devanagari and its neighbours picked up a series of adjustments over the centuries after Siddham left India: a dot under a letter to mark the retroflex flap, another to mark a borrowed f, another for sounds taken in from Persian and Arabic, punctuation, and conventions for numerals and abbreviation. None of it reached Siddham, because by then Siddham was somewhere else.
What was preserved in East Asia was the script as it stood when it was received, kept in use for a purpose that never required it to write anything but Sanskrit. It did not have to accommodate a changing spoken language, a new administrative vocabulary or a foreign literature, so it did not change in the ways a working script does.
The result is a script that is unusually clean as a witness and unusually narrow as a tool. It records the classical sound inventory precisely and it cannot write a modern Indian language properly at all. That is worth saying because it cuts against an easy assumption: Siddham is not an ancestor of Devanagari that stopped halfway, it is a sibling that was taken out of circulation and kept, and the differences between the two are largely things that happened to the other one after they parted.