Question about p-groups

Whenever I have seen classification theorems for groups of order pⁿ for a prime number p, every group is classified as some combination of cyclic groups of order p^k through semi-direct products and direct products. Is there some theorem or conjecture explaining why extensions on p-groups seems "more limited" in some sense? (It should be made clear that I'm basing this is off of what I've seen from n=1 to n=4, so it's possible this is just some law of small numbers kind of thing)