I feel they likely have so embedded themselves in daily life in various and assundry locations they are "taken for granted" as part of the typical daily grind. If the locations they are planning to use are intertwined along that route, who's to be the wiser if they do or do not pull off their attacks? Packages or bags left under bus seats or on benches or ledges...even fast food bags could be lethal if packed with incendiary devices (besides the food).
Of course cost is prohibitive for preventive equipment, but the potential loss of human life has great cost as well.
True, that is what they want. However, there is a degree of nervousness that accompanies an operation. If people are getting arrested, the potential that their operation is compromised would likely be strong enough to lay low and reschedule once the coast is clear and people are not thinking about terror are they are at the present.