Why is genetic drift undesirable in a population?
Genetic drift occurs in all populations. Let's look at seed corn crops:
It may not be desirable if the outcomes are genes that are not useful.
If the population (corn crop) has large and full ears, the individuals with fewer kernels will not be desirable. These could not be used as seed corn for the next year.
If this drifting continues and more and more undesirable ears form, the entire field would have to be plowed under.
The farmer will be looking for stable and uniform ears. Some strains may not work out and new varieties will have to used. Those things take time, money and research.