It's mountainous territory. Power and infrastructure moves through Serbia. The only way around is to get from Hungary to Romania and down to Bulgaria. The roads there are not optimal, however.

I'm just guessing. I don't know. Others have mentioned oil and gas pipelines which need to scoot up into Austria or empty into the Adriatic.

This still doesn't explain why Germany was so quick to recognize when all the other countries were warning them not to, including the USA.