Given that the Independent Sacramental Movement is so small and scattered, why do some choose to be part of it rather than turning to Rome or Canterbury?
For many, the ISM is precisely where they've turned after being rejected by Roman Catholicism. People who are divorced and remarried, lgbtqia+, or women with a calling to ministry are not truly welcome as they are, and affirmed in their identities, by the Roman Catholic Church. Within the ISM there are many very welcoming jurisdictions where they can be true to their Catholic faith and true to themselves. Now, mind you, not all ISM jurisdictions are so welcoming. Many are, but some few are ultra-conservative. Care must be taken in choosing an ISM jurisdiction that aligns with what you are seeking. This is usually resolved with a careful review of the jurisdiction's website. In any case, an ISM parish will typically have a mass that is familiar to a lifelong Roman Catholic, with some variation possible, but where everyone can take part fully.
The Episcopal Church is certainly a valid alternative for Roman Catholics, given its liturgy and progressive nature. However, as similar as the service may be to the mass, there have been hundreds of years of evolution in the Episcopal liturgy. This has given it a different feel from the Catholic mass that some might not find to their liking. Also, the Episcopal Church generally doesn't offer the sacrament of reconciliation or consider other rites to be sacraments as such either.
The ISM often offers a familiar liturgy, set of sacraments, and theology that former Roman Catholics find appealing. Given that, it's not surprising that many Catholics opt for the ISM when presented with the options.
