Why is there exactly one non-solvable group of order 180?

Let $G$ be a non-solvable group of order $180$.

Let's count the Sylow 5-subgroups of $G$. There can be $1$, $6$, or $36$.

If there's $1$, it's normal (call it $P$), and $G/P$ is a (solvable!) group of order $36$, so $G$ is solvable; contradiction.

If there's $36$, then a Sylow 5-subgroup $P$ is self normalizing, and being abelian, this implies $G$ has a normal subgroup of order $36$ (Burnside's transfer theorem); again, this implies $G$ is solvable. [Note that the argument here also implies if $N_G(P)$ is abelian, then $G$ is solvable.]

So $G$ has $6$ Sylow 5-subgroups; if (again) one is called $P$, we have that $|N_G(P)|=\frac{180}{6}=30$. Since this is twice an odd number, and all groups of order $15$ are abelian, we see that $|N_G(P)/C_G(P)|\le 2$. Thus if $x\in N_G(P)$ has order $3$, we also have $x\in C_G(P)$.

The conjugation action of $G$ on the Sylow 5-subgroups maps $G$ to $S_6$, and since no element of order $3$ centralizes an element of order $5$ in $S_6$, we get that the subgroup of order $3$ in $N_G(P)$ is in the kernel of this mapping $G\rightarrow S_6$. Since already then the image has size at most $\frac{180}{3}=60$ (and $G$ is non-solvable!), this must be exactly the kernel. So $G$ has a normal subgroup of order $3$; call it $N$.

[The above is just a long-winded argument to find a normal subgroup of order $3$.]

Note that since $G$ in non-solvable, for any non-trivial normal subgroup $K\lhd G$, either $|K|= 60$ or $[G:K]=60$. In particular, consider $C_G(N)$. It contains a subgroup of order $9$ (in fact it contains all of them), as well as $N_G(P)$. So it has order at least $90$, and we conclude $C_G(N)=G$, otherwise stated as: $N$ is central.

Now suppose $G$ is perfect, i.e., $G'=G$. If we let $Q$ be a Sylow 3-subgroup (which of course contains $N$), we can form the transfer map $\phi:\ G\rightarrow Q$. Since $G$ is perfect, $\phi(G)$ is trivial. But if $N=\langle n\rangle$, then $\phi(n)=n^{[G:Q]}=n^{-1}$, because $n$ is central; contradiction.

Thus $G' < G$. We don't have $G'=N$, since $G/N$ is non-abelian. Thus $G'$ has order $60$. We can't have $N\le G'$ (it implies $G$ is solvable), so we have $G'\cap N=\lbrace 1\rbrace$. In particular, $G=N\times G'$, with $G'\cong G/N\cong A_5$. We are done.

[I should note things would go a lot quicker if we used some group cohomology, or even the Schur multiplier of $A_5$.]

I think it is fairly easy, though perhaps a little lenghty and annoying, to show that every group of order less than 60 is solvable, thus making $\,A_5\,$ the smallest non-solvable group, thus making every finite group of the form $\,A_5\times H\,$ non-solvable.

Now, if your university's library has access to JSTOR, you have a paper determining all the (37) groups of order 180 here http://www.jstor.org/discover/10.2307/2371201?uid=3738240&uid=2129&uid=2&uid=70&uid=4&sid=56184846333