If you want useful public transit then it needs to connect population centers where people are. People are lazy and don’t want to walk more than 1/2 mile to a bus stop so if you have a population density of 1000/ sq mi that means any one bus stop is only going to be able to provide adequate coverage to 250 people. With so few people per stop it needs to make a lot of stops to be useful which then makes it slow which further lowers use. At that density it also doesn’t make logical sense to have designated bus lanes so they are stuck going slow in traffic as well. So now you have an expensive system that nobody uses because it sucks

If you have higher density then you can justify more lines which makes them actually useful and can add things like light rails which really make a difference

Bike transit is usually easier in those lower density areas but due to the low density getting between places is usually a bit further away so there are usually higher speed limit roads that aren’t as good for cyclists so more expensive barriers need to be constructed or they have to follow less direct paths which causes cycling to be slow