Because ketosis is forcing your body to create glucose from protein, which is a very energy-demanding process (that is good), but it's also extremely hard on your kidneys.

It is possibly fine to be in ketosis for a little while (and it can serve a medical purpose), but you should absolutely not keep it up long-term.

As with all crash diets where a lot of weight is lost rapidly, the weight is very likely to pile back on when the diet is over.