In the US, the police do not generally need a warrant to tail you as you go around town, but it is phenomenally expensive and difficult to do so. Cellphone location records, despite largely providing the same information, do require warrants because it provides extremely cheap, scalable tracking of anyone. In other words, we allow the government to acquire certain information through difficult means in hopes that it forces them to be very selective about how they use it. When the costs changed, what was allowed also had to change.