Number one is an image problem - they aren't sexy. Here in the States, we are obsessed with the 'European' way of life. Most major European cities have some form of streetcar system or underground metro, if not both. To us, those are sexy. Nevermind that those cities have buses, too.
The bus _could_ be more sexy. BRT with dedicated lanes, priority signaling, clean & modern interiors, at-grade egress, and most importantly, consistent and reliable service. Those changes would go a long ways towards making the bus 'cool' enough to ride.
The other is that because buses are inherently at least a little inconvenient, there has to be an incentive to use them. For most people, the main driver is financial. Ten years ago when the economy was shaky, I, a consistent multi-modal commuter, saw a huge uptick in the number of people riding the bus. When the economy got stronger and more surefooted, buses started to empty out again. Now I see fewer and fewer people of 'means' on the buses - the ones that remain likely don't have other options. When people can afford _not_ to be inconvenienced, they won't be - whether that means buying a parking pass, ponying up for gas & insurance, or taking rideshare everywhere.