One reason is that when growers compare the cost of cultural alternatives to chemicals, the cost of chemical control is often much lower than the cost of alternatives.
In many cases, we really don't know the cost of alternatives. How much does it cost to fallow a field for a year? This would include the cost of weed control which would typically need to be done several times during the year, and in some cases, the cost of erosion control. If one is only going to grow a crop every other year, then the crop grown will need to yield the grower more than twice the rate of return of one grown every year utilizing chemical control.
If one examines the cost of organic produce in their local supermarket, it is obvious that costs are higher, but not usually twice that of produce grown with the use of chemicals. This would indicate that growers are not receiving twice the value for the growing of organic crops.
In many cases, the use of a cultural alternative will have a lower efficacy than the use of a chemical method. The differences would depend on both the cultural method and the chemical method. This means that one would have to utilize more than one cultural method, or perhaps a combination of cultural and physical methods, inorder to achieve control comparable to a chemical.
Another problem is that of putting cost figures on the environmental damage that is attributed to the use of chemical nematicides. At the present time, what ever these costs are, they are being subsidized by society.