Why did the organ player disappear from the rock band at the end of the 60's?

Seems like every band had an organ/synthesizer in the 60’s but toward the end of the era the organist/piano player fell by the wayside and by the mid 70’s was a thing of the past in most bands.

Was it an integral part of the psychedelic sound and disappeared as rock music went in search of new and different sounds and styles?

About the same time it seemed that the size of bands began to shrink and four was, by default or by design, chosen as the optimum number of members needed. Was it and economic choice or stylistic choice that went along with this reduction in band members?

I realize that none of this is a universal truth and that there were bigger bands, smaller bands, bands with synthesizers and organs and even trumpets (although that ain’t what we call rock and roll) but for the vast majority of bands rock became synonymous with a drum set, base guitar, lead guitar and rhythm guitar with one or more also taking up singing duties.