Why are teleosts more abundant and diverse than sharks?

Abundance:

In any ecosystem there is a balance of predators and prey. Sharks are what we call apex predators, they have no natural enemies. This means that their maturation rate is slow, they reproduce slowly, and they live a long time (in general). These are all reasons why they are unable to cope with the modern human induced pressures of fishing and finning.

Anyway, the ecosystem is a triangle of total biomass . This means, for example, that for every 100 antelopes on the African savannah, you have 10 lions. Or 50 antelopes would be enough to only support 5 lions and so on. Now imagine a more complex ecosystem- say a leopard seal, a penguin and a fish. You would need a 100 fish to feed 10 penguins which would feed 1 seal. Or 200 fish would sustain 20 penguins which would provide enough food for 2 seals. (All numbers are approximate!)

So, teleosts and sharks work in the same way. Sharks are predators with no natural enemy and so are at the top of the triangle. Telesosts have numerous predators and so are in the middle of the triangle. They need to be far more numerous than their predators but less numerous than their prey. The creatures at the bottom of the triangle that get eaten by hundreds of different predators have to be the most numerous in order to cope with the predatorial pressures. Think how much grass is needed to sustain a herd of antelope!

Diversity:

Teleosts are found globally. Speciation occurs for a number of reasons including population isolation and adaptation to environmental pressures. So the more numerous and wider spread you are, the greater the number of species will occur. They also speciate based on predatorial pressures, climate, disease, mutation, water depth, salinity, and so on. Also individual populations may appear different even if they are the same species for example Bonnethead sharks that live in the shallow waters of Florida are slightly darker than elsewhere because they get suntanned!

Shout if any of that is not clear!

Okay, I'm nit-picking here, but...

If you're thinking about the numbers of predators and prey in an ecosystem, the dominant control is likely to be the amount of energy in the system (or more specifically, the amount of energy lost as you move up through the food chain).  Most of what a herbivore eats goes into fueling its daily life, with only a little bit being incorporated into its body.  Therefore only a small proportion of the energy it takes in will be available to a predator for food, and overall a given number of prey will always support a smaller number of predators.  If you add in an extra predator on top of the food chain, then there is even less energy available, and the population of this predator will be even smaller still.

So, we would expect to see the trophic pyramid that Emma has described, with lots of herbivores and few top predators, but (as far as I understand things) it comes down more to the fact that there is insufficient energy in the system to support larger populations of predators, rather than animals in lower trophic levels having to be more numerous to withstand the intensity of predation.  I suppose you could think of it as a "bottom-up" rather than "top-down" control.  And it's why, in the words of Paul Colinvaux, "big, fierce animals are rare".