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As mentioned in the sibling comment, different frequencies means probing different things.

I found this talk[1] rather nice to explain the NANOGrav experiment, and at 6:45 there's a very nice plot that shows where NANOGrav fits in compared to the other gravitational wave experiments and which type of sources the various experiments can probe.

[1]: https://pirsa.org/20100068

LIGO detects gravitational waves in the frequency range of hundreds of Hertz (10^2 Hz), which are produced in the last moments of the in-spiraling of merging neutron stars or blackholes.

Nanohertz are 10^-9 Hz which relate to a rotational period of decades of years.

https://en.wikipedia.org/wiki/Gravitational-wave_astronomy

For one we didn't have a way to measure the speed of gravitational waves before [1].

[1] https://physics.stackexchange.com/questions/622729/did-ligo-...

Those two methods observe waves of much different wavelength/frequency, and the mechanisms that create such different wavelengths are assumed to be different. So cosmologists are studying different things by looking at different wavelengths.