I would blame how Austria, a very small country, is organized into 9 provinces that actually have their own budget and can pass their own laws on some topics.
Rail service is funded at the federal level, so there's less arguing about who pays for what. Bus service, however, is managed by regional transport associations funded by the provinces. This creates disincentives for cross-province bus routes because no single province wants to pay more than its 'fair' share for a service that primarily benefits voters in another province.
Similar dynamics play out at the city/province level. Take Linz, the provincial capital of Upper Austria: the city has had a social democratic (SPÖ) mayor continuously since 1945, while the province has had a conservative (ÖVP) governor for exactly the same period of 80 years. This disincentivizes the province government from helping to fund public transport within or into the city, because it's a win for social democratic city voters, while the more conservative rural voters would rather take the car anyway since they often can't do the whole trip by public transport.
Arguably the reason for the excellent public transport in the city of Vienna is that they are also their own province. Their mayor/governor, who has been a social democrat as well for the last 80 years, always controls both levels of funding.