As per title, what is/are the reason(s) Sonnet doesn't have integration with Synopsys Custom Compiler, while having seamless integration with Cadence Virtuoso?
There's no specific reason, except for priorities.
During my time as a Sonnet reseller, I had been pushing Sonnet to implement a Cadence Virtuoso integration, because we had large customers requesting that. The Sonnet software architect (Mike K.) who implemented the Cadence interface was also in touch with Synopsis (later) but obviously other items on the Sonnet roadmap (e.g. tech layers) where considered more important.