It`s not,that`s the thing,Jill had what 5 min max during ep1-2,while proudly wearing the label of an "LI"-the only thing providing such label was the fact of having relationship points.
The existing side girls already are part of the story,we already have some info on them and in some cases basis for possible relationships.Given Quinn for example all she`s missing,really,is said points.
Now the time that will be devoted on adding new side flicks,anyways,could have been simply transfered to existing characters,as you noticed yourself,even when it comes to "the main girls" some of them have lil time as it is.
And those "one and done" chars are not really one and done,Envy and Rose will have a comeback,Lilly aswell - wouldn`t you rather to have 5 more scenes with existing girls,instead of the strip club?Because I would,I would much rather to have phone content with Quinn than with Envy,I would rather to have 1 of the HoT girls be a reward instead of Rose.
In other words,not extend development time even further but relocate it to existing characters,rather than adding new ones.