Australia

It does? It seems completely obvious to me. Buses are like cars, but bigger. Cars have huge numbers of complex parts that need to be maintained, but the most obvious one is tyres. Rubber tyres wear out, and on heavier vehicles they wear out faster. Fossil fuel–powered buses additionally have very complex engines and transmissions which require significant amounts of maintenance for which there is simply no equivalent on trains. Electric buses perform better in this capacity, at the cost of being heavier and thus putting more wear on their tyres. Because of their maintenance needs, you'll need to over-purchase buses in order to have the required number running while others are off the road for maintenance.

There's also the secondary effect that buses do a lot of damage to roads, being both heavier and more frequently accelerating & decelerating at the same locations than single-occupancy cars, and thus you end up needing to spend more money on road resurfacing. And again, EVs end up worse in this regard than petrol, diesel, or natural gas vehicles.

Trains are steel on steel. They wear out shockingly little. Their electric motors require less maintenance than ICE engines. And the vehicles themselves last a lot longer due to this simplicity, so you can buy trains now and keep using them for far longer than you can keep using a bus you buy now. I'm not clear on the lifespan of electric buses, except that at a minimum the battery will need to be replaced much more regularly than a train would need replacing.