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.

To move the same number of people you need several times more buses, which means more engines that can fail, wonky acceleration profiles from the variability of road traffic conditions wears engines and physical road conditions wear tires. The lifetime of a rail system is at least 3x as long as a bus. Many metro systems across north america are still running some trains from the 80s that are only just being replaced. Chips and burrs on metal wheels can easily be ground down then replaced after many, many repair cycles, whereas worn or blown tires can only be replaced when the train wears.

So you're right, buses can can be set up faster and are more versatile on our existing road networks and lower upfront capital costs. In the long term, trains are cheaper to maintain and operate and provide a better rider experience.

https://www.youtube.com/watch?v=W0gZPTC15M0