Why are transition metals often used as a catalyst?

Answer 1

short answer: because they work so well.

slightly longer answer: they are big enough to catch electons or give electrons as needed, but can be separated from the reaction easily. given big enough ligands they can be used to create a desired steriochemistry which makes a synthetic organic chemist's life a lot easier.

Answer 2

Transition metal atoms have 5 degenerate d orbitals. This provides a lot of space for electrons to transit between almost same energy levels, thus they are used as catalysts.

Answer 3

Transition metal atoms are large and they crystallise out in differrent definite space lattices with a lot of empty space or voids.These voids have characteristic shapes and this where the specific reactants can interact.They also weakly attract the reactants as they have incompletely filled d sub shell.

Answer 4

They have d-orbitals that match the energy levels (and symmetry) of HOMO/LUMO's of hydrocarbons. This facilitates electron transfer...and thus chemical reactions.

That's the short answer.