This is a great question. Like the middle child in a family (I am one), the mid-cap stocks are often forgotten. According to Investopedia, a Mid Cap stock is A company with a market capitalization between $2 and $10 billion, which is calculated by multiplying the number of a company''''s shares outstanding by its stock price. Mid cap is an abbreviation for the term "middle capitalization".
Some examples of these mid cap companies are Dillards, Cabelas and American Eagle.
Here is the answer to your question. Simplicity.
Most model portfolios are put together because they want you to invest with them. If they make it too complicated, then you will freeze and not invest. By giving you fewer choices, you are more likely to invest.