It's design efficiency and design wisdom.
The only time TTL level sync presented itself as a requirement was in regards to the OSSC / F3. It made more sense to build an external option around that than to add complexity and cost to the main product for the sake of ensuring compatibility with a single use case.
Furthermore, the OSSC hardware hack wasn't available at the time the Minigun was being created so there was never any compelling reason to cater to TTL sync over the Mini DIN 8. Quite the opposite in fact, there was a need to maintain safety and adhere to standards rather than allow an end user the opportunity to feed TTL level sync through a scart video cable where safe levels were expected.

The above sync booster is an effective and easy way of enjoying F3 over an OSSC. As mentioned, at the time it was mandatory to utilise the VGA port of the OSSC in able to utilise TTL sync and besides this, for some it's a preferred option as opposed to hardware modding your OSSC.
Design flaw it is not.
The only time TTL level sync presented itself as a requirement was in regards to the OSSC / F3. It made more sense to build an external option around that than to add complexity and cost to the main product for the sake of ensuring compatibility with a single use case.
Furthermore, the OSSC hardware hack wasn't available at the time the Minigun was being created so there was never any compelling reason to cater to TTL sync over the Mini DIN 8. Quite the opposite in fact, there was a need to maintain safety and adhere to standards rather than allow an end user the opportunity to feed TTL level sync through a scart video cable where safe levels were expected.

The above sync booster is an effective and easy way of enjoying F3 over an OSSC. As mentioned, at the time it was mandatory to utilise the VGA port of the OSSC in able to utilise TTL sync and besides this, for some it's a preferred option as opposed to hardware modding your OSSC.
Design flaw it is not.