> Roads are physical easy to understand

???

Yes, roads require less 'active upkeep', an austerity government might stop funding transit but they will not close roads. However, this is more of a feature of the supposed 'independence' of car transport, not 'easy to understand'.

In addition, defunding transit would be a lot harder and weirder if it is in a form that requires actual infrastructure (metro, train, tram) rather than some stops and a schedule (bus). That's one potential reason to trust bus transit projects less.

Do keep in mind that roads aren't free after construction either; they typically require pretty significant upkeep (especially things like repaving, but also e.g. traffic signals).