Self employed

The only answer to your question is because that is the way the laws regarding home office expenses have been written.  

If there is not enough income attributable to the home office versus other business locations, then the home office expense is either limited or eliminated.  The only factor within the taxpayer's control here is to determine whether the home office was used more or less with regard to the income earned from the business.  

Take a look at the examples from the TurboTax help content for further details about how this limitation is applied:

Income Earned from Home Office

The IRS limits the total of certain kinds of home office expenses - the ones you would not be able to deduct anywhere else on your tax return, such as your utilities and repairs - to the income earned from activities in your home office. Although these expenses cannot themselves create a loss on your business, you can carry over any unused portion to next year's home office deduction.

Here's how it works. Let's say

 - Your business income before any expenses was $10,000

 - All business expenses that would not limit your home office deduction were $4,000, and

 - The home office expenses were $3,000

1). If 90% of your income came from business conducted in your home office, then you can deduct all of your home office expenses:

 - $10,000 X 90% of income from the home office = $9,000 from business use of the home

 - $9,000 - $4,000 other expenses = $5,000 available for home office expenses

 - $5,000 is greater than $3,000 home office expenses, so you can deduct all of them.

2). However, if 60% of your income came from business conducted in your home office, then your home office expenses will be limited:

 - $10,000 X 60% of income from the home office = $6,000 from business use of the home

 - $6,000 - $4,000 other expenses = $2,000 available for home office expenses

 - $2,000 is less than $3,000 home office expenses, so you can deduct $2,000 of them this year, and carry the remaining $1,000 to next year.

3). In addition, if 30% of your income came from business conducted in your home office, then you would not be able to deduct any of them this year:

 - $10,000 X 30% of income from the home office = $3,000 from business use of the home

 - $3,000 - $4,000 other expenses = $0 available for home office expenses (this will never be less than zero)

 - You cannot deduct any home office expenses this year, but you can carry the entire $3,000 to next year.

@gregcg