All you have to do is listen to the final movement of Beethoven's Ninth Symphony or Handel's Messiah - two classical pieces that are played all the time on classical radio.
Truthfully, there probably was some separation for a long time of the mindset of using vocals with music - much of that music was originally composed for the church. Many of the Oratorios follow this trend.
Another fact was when composers wrote pieces for vocals, that meant bringing in a choir to sing along with the orchestra - not always easy. Early on most vocalists would be drowned out by a chamber ensemble, then especially with symphony.
Some composers admitted they didn't want to write for the human voice.
It really has only been since the 1960s that classical music stations cut back on programming opera and other vocal selections, at least in the U.S. - probably in part because most of the vocal music is not sung in English, and Americans tend to be xenophobic about other languages. You also had the development of Opera shows, which carry weekly performances of Opera companies.
Now, if you listen to the BBC or classical music in other countries, thats not the case, you hear plenty of vocal music.
By the way - Mozart, Beethoven, Bach, Schubert, Brahms - all giants of classical music - wrote many pieces that included vocal selections - and they didn't considered them any less skilled pieces - they simply added the voice as another instrument to perform their work.
By the way - many critics list Beethoven's Ninth Symphony as the greatest work of classical music. Definitely argued - but it shows you how highly thought of the piece is - with a vocal movement.