I'll say right up front that I've never played on a 7 foot table, nor for that matter on an 8 foot table as far as I can recall. To me, a 9 foot table was what pool was all about and anything else might as well have been bumper pool. So in looking for a table for my home I've never considered anything but a 9 footer, which with a room appx. 20x16 should be just fine.
But I am curious about why the 7 foot tables seem to be so popular, especially the Diamonds.
To a very large degree, people who actually play pool and want a table in their home buy as big a table as will fit. People who live in smaller houses will gravitate towards 7 footers, people in somewhat larger houses will gravitate towards 8 footers. People who have either a larger house or a large room which can be dedicated to pool gravitate towards 9 footers. I have an 8 footer because that is as big as a rom I have to dedicate to pool.
Bars gravitate towards whatever brings in more money per unit time.
Often these turn out to be 6.5 foot Valleys.