Why are bipolar transistors used for switching applications?
In general, FETs require much higher drive voltages than BJTs. Let's say you have a circuit running on 5 volts, and you want to switch a relay. If you get a 5-volt relay, you can use a BJT and everything will work off of 5 volts on the control side. The base voltage of a BJT is only about 1 volt, so you have about 4 volts to work with in developing the base drive. Granted, you may have to provide a lot of base current (since BJTs typically want to be switched at a base/collector current ratio of about 1:10).
If you're not using logic-level FETs, you need to provide something like 10 to 15 volts on the gate to guarantee the FET is switched fully on. Doing this with a 5 volt supply (or even a 9-volt battery) is not something that's going to work out well.
So, beginner circuits will typically operate from a single, relatively low supply voltage - and that gives an enormous advantage to using bipolars.