My personal thoughts on the reason not all sizes are included with all quilt patterns: Depending on the quilt design, it can take a large amount of time and space to write and publish multiple sizes for a quilt.
For an all-over repetitive block pattern, it's fairly easy to just increase the number of blocks and the size of the borders. Often they just have the yardage requirements for larger sizes added at the end of the pattern or as an "online extra" somewhere. These are also the easiest patterns for most quilters to enlarge on their own. This is true for template patterns as well. Just enlarge the templates.
For a more contemporary pattern, and those not based on blocks, increasing the size can mean increasing the measurement of every piece cut for the entire quilt. That would almost require writing an entire new pattern (and using up costly magazine space). Simply giving the larger yardage requirements without the altered cutting, and possibly construction, requirements would be quite confusing (which may result in lost sales).
Personally, I think the cot/lap/throw/couch/car size is the most versatile and the most visible, in addition to being the easiest to actually finish, 'grown-up' quilt. More people will see and use a quilt that size, and more often, than they will a bed quilt. It's the only functional size quilt I design/make any more for all those reasons. Mind you, all of the above thoughts are just my humble opinion from a design/marketing standpoint. (sorry to be so wordy :oops: too much coffee)