Couldn't they have just liked the song?
If they were a Holiday Inn lounge act, that would be all the motivation they would have needed.
But for a group that was selling millions of records two years before, the criteria for what to record and release needed to be a little more solid.
They'd watched the peak chart positions for their singles go from #4 for "The Look of Love" to #6 for "Fool on the Hill" to #16 for "Scarborough Fair" to #62 for "Pretty World" to #66 for "(Sittin' on the) Dock of the Bay" to #95 for "Wichita Lineman" to not even charting for "Norwegian Wood" (which is what happened to "For What It's Worth").
And they'd seen the album sales (in those days driven by singles play on the radio) head south as well (FOOL ON THE HILL #3; CRYSTAL ILLUSIONS #33; YE-ME-LE #71; STILLNESS #130).
Fail to turn that decline around and you'll lose your recording contract...which is exactly what happened to Sergio.