Simply because the cash that the company receives (the proceed) from the sale of fixed (LT) assets is considered as a cash inflow in the Cash Flow From Investing Activities (FCInv). So, since we are using the Indirect Method for calculating cash flows, NI is already carrying the gain or loss in the sale of those LT assets. Have you seen it yet?
This formula FCFF = NI + NCC +Int(1-T) - WCi - FCi is considering NI and FCInv at the same time, so if we don’t adjust the gain or loss into NCC, we would be double-counting the gain or loss because obviously the cash proceed of the sale already includes the gain or the loss 
First of all, how the accounting gain or loss is calculated? The balance sheet carries the asset at depreciated value or fair value (market value). Suppose we sold the company transport truck for 1,000 and the balance sheet value of the truck was 800. So we made a profit of 200. The 1,000 cash will go to FCInv as +1,000 inflow and the 200 gain will go to the Income Statement as a gain from sale of LT asset in a line below operating profit (because this is not an operating revenue, but an extraordinary income).
Note the 1,000 amount includes the gain as I said above !
If we calculate FCFF without considering the subtraction of the 200 gain as negative NCC, we would sum a NI that includes a 200 gain and a FCInv of +1,000 that also includes the 200 gain. Double counted the gain.
In case we want to calculate Operating Cash Flow (CFO), the adjustment is the same, we also deduct the 200 gain. But the reason this time is that that gain comes from a non-operating activity, so we exclude the amount.
The same analysis apply for the case when we sell the asset at loss, the signs are reversed.
A numeric example:
Continuing with the truck example, also assume NI is 500, there are no other NCC than the gain in the sale of the truck, interest expense is zero and Working Capital Investments are zero (WCi).
FCFF = 500 - 200 + 1,000 = 1,300
Remember that 500 already includes the 200 gain, so I subtract the amount and let the 1,000 carry the gain.
Any questions, please ask