It seems like most people are using Qiskit, what's the difference?
The answer above, from a Cirq developer, is mostly in the right direction. We (Qiskit / IBM) probably do spend more on outreach and education (I also do not know for sure). Democratization of quantum computing is a primary goal in what we do. Basically, the more doors you open, the more people can walk through and do cool things. Free access to quantum hardware is a key component of this as well. Of course, the user base also includes a large group (100+) of academics, startups, industry, and government partners as well. As such, Craig's answer is only part of the bigger story with respect to number of Qiskit users.
Google was supposed have public access to their systems in the first half of this year (2021), but that has yet to materialize. I would assume that the usage of Cirq would increase once this occurs. Google is also hiring in the space of quantum education (here) which I guess means they are looking to do similar things as IBM at some point. Cirq does have other users as well. They currently have cohorts of academic groups who utilize their hardware. Cirq is also used in the startup space. The non-profit Unitary Fund's Mitiq (here) comes to mind as an example. There are also several commercial engagements that Google has with industry that one assumes makes use of Cirq. Again, the user base of both Cirq and Qiskit is much richer than the above answer seems to suggest, and getting a handle on the actual numbers of both is a challenge.
There are also more options for accessible hardware than listed above. IBM does give by far the most free access, but there are others out there. IonQ gives some free access to select academic groups, and AWS and Microsoft Azure give out credits for usage. Again, these usually target academics and/or winners of hackathons, etc. It is also possible to pay for access on a pay as you go, or bulk buy, basis through several cloud providers (Google only offers IonQ and not their own hardware). It is possible to program most hardware through either Cirq or Qiskit. However, I would agree that for most people the quickest, and cheapest, way to get started learning and/or doing research is with IBM and Qiskit.
Finally, one can look at a variety of metrics to gauge user interest, but none tell the full story, and it is easy to cherry pick ones that give you the result you want.
Completely speculating here, but I think the main differentiator is that IBM has a stated goal of, and spends significant resources on, getting quantum computing into everyone's hands. For example, they run a lot of tutorials and create a lot of course materials. And, maybe most importantly, they make it reasonably frictionless for someone to run something on their quantum computers.
Cirq does spend some resources on tutorials and course materials, but they are far less emphasized. And access to google's quantum hardware is much more controlled.
So, suppose you're a student. You want to run something on a quantum computer. Your options are: use IBM's (so learn qiskit) or... well, that's kinda it. Also your professor gave you an assignment to do that. So you go read up on using qiskit.
I will add one note of caution though. Be cautious of "it seems like". Programming communities tend to make little bubbles. You should try to get some concrete numbers. E.g. the number of "github stars" is pretty similar between the qiskit github repo and the cirq github repo. But the number of stack exchange questions is 10:1 in favor of qiskit. Maybe that's something to do with students being more likely to ask questions publicly? I dunno.