Step 1: What is an alloy?
An alloy is formed when atoms of one metal replace atoms of another in a crystal lattice without drastically distorting the structure.
Step 2: Condition for alloy formation.
For this substitution to be possible, the atomic radii of the metals must be similar (within about 15% of each other), so the lattice geometry is preserved.
Step 3: Why transition metals satisfy this condition.
In the transition series, each successive electron is added to an inner $d$-orbital. These $d$-electrons shield the outer electrons effectively, keeping the atomic radii nearly constant across the series.
Step 4: Conclusion.
Because transition metals have very similar atomic radii, their atoms can freely substitute one another, enabling alloy formation (e.g., brass: Cu-Zn, steel: Fe-C).
\[ \boxed{\text{Similar atomic radii}} \]