why is 0w20 conventional oil not available.
Because its impossible to make a 0W20 oil using current technology with only conventional lubricants. Even semi-synthetic blends are far and few between, although we will probably see more of them as 0W20 is getting more prevalent.
When you formulate an oil like this, you start with base oil that will reach the "0w" cold visc requirements. This part can be done with conventional oil.
However, you have to then prop up the KV100 and HTHS to reach the "20" requirements, using VII chemicals. Still, this all can be done with conventional oil.
The last step is then passing all the high-temperature tests like NOACK, piston deposits, TEOST, etc. which really needs some GroupIII and/or GroupIV synthetics to pass that part.