Before I answer your question, this is the BBC orchestra performing the Missa Solemnis with soloists in front.
To answer your question, I will ask another question: Why are there dozens of different wind ensemble setups? Why are there different setups for orchestras and choirs? The answer to this question is that each setup creates a different sound and allows ensemble members to hear the one another different.
Different ensemble setups are also created for different group skill levels. You wouldn't put a middle-school choir in a mixed quartet setup; nor would you have them sing 4-part music for that matter.
Soloists are often put at the front of the stage for clarity of text, projection, and balance - that's why orchestras play in pits for opera. If the soloists are put elsewhere, it could be for dramatic effect or to take better advantage of the space. One great example of this is Benjamin Britten's War Requiem where in the premiere at Coventry Cathedral the two male soloists were placed on one side with the full orchestra and the female soloist was placed with the children's choir and chamber orchestra on the other side.
If space is cramped, there may not be anywhere for the soloists to go except squished by the orchestra unfortunately. If space is not cramped and there is no viable reason for them to be back there, then it is an oversight of the conductor and director as they will be that much harder to understand.
With respect to the Missa Solemnis specifically, Beethoven was well into beginning to lose his hearing, so I doubt it mattered much to him anyway - especially because he never actually heard the entire piece premiered in his lifetime.
I am not aware of any specific historical practice with respect to placement of soloists in front of the choir.
Hope that helps.