I believe it's because its a bandwidth hog. You can make more people show by changing the time in Admin CP-Configuration-Features and Options-Basic Settings-User Online Time Threshold
Change the number of minutes to 1440(if I remember correctly) and it will show members that have visited in the last 24 hours.
Ok that is not really a good idea. The "Users Online" right now is useful for seeing who is actually there, in case you need to contact them. It's a great way to see your average activity at any given time.
A users online today list shows the total amount of
members who have been online in a given day. That's a good way to see your overall activity level per day.
The little number that says "X users have been online today" is really not the same thing because that number factors in guests...