Put simply, because an embryo is very very small so it's easy to freeze it quickly. Freezing quickly is vital in living things we wish to keep living because ice that forms slowly creates big crystals that tear apart cells. Defrosting cells that have been torn apart by ice gets you something very dead. Try it with a cucumber in the freezer, you'll get a very floppy cucumber. Small enough things can be flash frozen and be fine because small things can have every part frozen quickly enough that no cells get damaged in the process. Adult humans though are very big, flash freezing a person would at best get you a pretty decent skin suit but all the internal organs would be mushy like the frozen cucumber.