The seismometers were part of the Apollo Lunar Surface Experiments Packages (ALSEPs), which were powered by radioisotope thermal generators. RTGs use the heat from decaying radioactive elements to produce power, and gradually produce less energy over time as the isotopes decay into more stable elements. By 1977 the ALSEP RTGs were only producing enough power to run either one experimental package or the radio transmitter at once; they probably would have stopped working altogether in a few more years anyway and it's better to have a controlled shutdown rather than a failing radio transmitter cluttering up the airwaves. Wikipedia also mentions "budgetary considerations", and apparently they wanted the ALSEP control room for another project as well.

Edit: I've just found [1], which adds a few additional details: first, they were designed for a lifetime of one year, so 1977 was well beyond their expected activity; and second, after they were decommissioned they continued to send carrier signals (but no data) which were used for various purposes.

[1]: https://www.hq.nasa.gov/alsj/HamishALSEP.html

More detail than you ever wanted to know can be found in the ALSEP termination report: https://nssdc.gsfc.nasa.gov/misc/documents/b32116.pdf