Why did "it" lose its initial 'h' but other pronouns such as "him" and "her" didn't?

The pronouns it, him, her had an initial h in the older forms of English which has been retained in her, him, but lost in it (formerly hit).

Etymology of it (Wikitionary):

From Middle English it, hit ( > dialectal English hit (“it”)), from Old English hit (“it”), from Proto-Germanic *hit

The word it had an initial h in both spelling & pronunciation but was then lost in Middle English onwards. Other pronouns such as him, her had also an initial h in Middle & Old English but has been retained.

Was this an irregular change or is there any explanation for the loss of initial h in that particular word?