TLDR: It's a soft spot for optimization around the ability to display 25 lines of text.
(And why this is important has been dicussed some time ago in an answer to your question about why 80x25 became standard)
Preface: As usual with such decisions there are many factors involved - and most of them are not hard but variable within a certain frame and in relation to others. So there might be more details than covered here.
For CGA one goal was the ability to display 25 by 40/80 characters. With 200 this alows 8 scan lines per character cell. Something that nicely fits with binary counters addressing character ROMs, as well as RAM and ROM size and/or programms calculating screen addresses.
It's the same why the Apple II got 192 lines as Woz was targeting a 24 line text dsplay. IBM in contrast needed to go for 25 lines to make it possible to have their mainframe screens displayed - the 3270 did offer 24 lines of user display plus one line of status information (*1).
Offering more than 25 lines of text (like 30 with 240 scan lines) would make programs converted form common 24/25 line displays problematic, as it's not always easy to support the additional screen estate. Where a Text processor may benefit, a 3270 session would look odd - at least.
While adding more text lines is problematic from a compatibility side, adding more scan lines per text line would have been doable. Like in 9 at 225 lines or 10 at 250 (*2). While allowing greater details on screen, it would require a more complex hardware (*3), increasing the cost. In addition it would break the nice 8 by 8 character cell used. Something that again simplifies hardware and programming.
While extending a scan line to 720 pixels might have been possible as well (like with more lines), it would again require more complex hardware - not at least even bigger character ROMs - muchlike with the lines.
Another way could have been adding a default blank scan line each 8 lines. While this may have improved text display (a bit) it would have made continous graphics impossible.
With different goals (no graphics, only text) IBM's MDA did use a 720 by 350 with 9 by 14 pixel per character on quite similar hardware. Just here focus was on a clear text only display.
So bottom line: 640/320 by 200 is a soft spot for optimizing a display arround text and graphics within the capabilities of displays based on TV components.
*1 - This 24 line user area of the 3270 is also the reason why Woz (and many other) did go for a 24 line text display.
*2 - 250 is still way within the ability of standard components. After all, CGA was ment to be displayed with a special made screen - just using standard (aka cheap) components.
*3 - Counting to 9 or 10 instead of 8 requires more complex counters an domparators unlike the easy binary power of 2^3 where just a coutner is needed - similar in software it would require a multiplication, while the value 8 just requires a shift operation. While the used 6845 can handle this, ROMs need to be larger - depending on the hardware design maybe even double the size to just accomodate a single additional scan line per character. Similar for an additional horizontal pixel per character, eventually resulting in quadruppled ROM size. And in full graphics mode addressing gets realy weired.