Could anyone give some pointers on why the unpure computations in Haskell are modeled as monads?

Well, because Haskell is pure. You need a mathematical concept to distinguish unpure computations from pure ones or to model programm flows in it respectively.

This means you'll have to end up with some type IO a that models an unpure computation. Then you need to know ways of combining these computations of which apply in sequence (>>=) and lift a value (return) are the most obvious and basic ones.

With these two, you've already defined a monad ;)

Could anyone give some pointers on why the unpure computations in Haskell are modeled as monads?

Well, because Haskell is pure. You need a mathematical concept to distinguish between unpure computations and pure ones on type-level and to model programm flows in respectively.

This means you'll have to end up with some type IO a that models an unpure computation. Then you need to know ways of combining these computations of which apply in sequence (>>=) and lift a value (return) are the most obvious and basic ones.

With these two, you've already defined a monad (without even thinking of it);)

In addition, monads provide very general and powerful abstractions, so many kinds of control flow can be conveniently generalized in monadic functions like sequence, liftM or special syntax, making unpureness not such a special case.

See monads in functional programming and uniqueness typing (the only alternative I know) for more information.