Why is there not just one or two curves that have been widely accepted as useful?
Several reasons; some of it is politics (for example, I suspect a good part of the reason behind national curves is, in fact, national pride), some of it is multiple teams working independently (for example, the Brainpool team, the NUMS time, the people behind the million dollar curves)
However, what can't be discounted is that people have different opinions on what makes a good curve. Here are some of the highlights:
What sort of prime is the curve based on. You can define an elliptic curve based on any finite field; recently, people have (mostly) settled on defining them based on large prime fields. For these curves, a large part of the expense is performing modulo operations based on the large prime. Now, if we select a prime with a simple binary representation, this modulo operation can be performed much faster than if we selected a random prime. On the other hand, some people note that these special primes make some blinding operations (important for side channel protection) harder, and so they do prefer random primes.
Lack of suspicious design decisions. If you look at the NIST curves, they are based on the equation $y^2 = x^3 - 3x + b$ where the value $b$ is a rather odd constant. Some people worry that $b$ might have been selected to induce an obscure weakness; that's a good part of the motivation for a number of the curves.
Curve equation. Traditionally, the curves have been based on the Weierstrass equation, $y^2 = x^3 + ax + b$ (for characteristic > 3), which can represent any curve (that is, if you have an EC curve formula, you can find a simple mapping from that to the equivalent Weierstrass curve). However, some people say that there are advantages to other curve equations (even if they can't represent arbitrary curves); Curve25519 is the most prominent example here.
On the other hand, if we look at what curves people actually use, there isn't that huge of a variety. We see P256 in wide spread use, we see some uses of P384 and uses of Curve25519 are becoming prominent. Every other curve would appear to be to be, at least, niche; used for an application or two, but not really that wide spread (at least, in my experience)