It is convention.
If the top number is a multiple of 3 and greater than 3 - 6, 9, 12... - the meter is compound and beats get subdivided by three and the lower number is the note value of the beat subdivisions.
So, 3/4 is simple meter, but 6/4 is compound.
...a time signature where there were six quarter-note beats per bar with an emphasis only on the first beat?
Technically you can't, because you specified the quarter notes are beats. To write that meter it would be 6/4. But, the 6 gets treated as compound. You want a meter that is both simple and has only a single accent on beat one. The only simple meters that do that are those with 3 as the top number: 3/2, 3/4, 3/8, etc.
If you drop the requirement that the quarter note gets the beat, then 3/2 will work. 3 half notes per bar, only the first beat is accented, subdivide the beats for 6 quarter notes per bar.
An overarching concept in metered music is accenting groups of 2 or 3. When division by 3 is possible, you potentially have a compound meter, a meter with groupings of 3, but you need the meter to define that. The classic case is 6 notes. It could be eighth notes in 3/4 or 6/8 depending on whether you want groupings of 2 or 3.
Any time you subdivide by 2 or 3 there is an implicit accenting on the first note of the group. Quarter notes in 3/2 are grouped in 2 and implicitly the second of the two will be weaker. Think of each beat as a down stroke and so the first quarter note of each pair is the strong down stroke and the second quarter note is the weaker up stroke. The beats are strong weak weak. But each of those has a weaker up stroke.
One strong beat followed by 5 weak beats does not follow the convention of grouping by 2 or 3. The system of meters is basically telling you to pick either of those groupings, but not allowing for a grouping of 6. Meters won't express that so you need special instructions of some kind. Unmetered notation probably would not be sure to work, because players will then look for patterns in 2 or 3. Meter 1/4 would be all accents, not exactly right. Probably the simplest thing would just be 6/4 or 3/2 and written instruction to play no accents accept on beat 1 or where an accent mark is given.