Step 1: Large, measurable values.
For a given concentration, osmotic pressure gives a much larger measurable value compared to the small changes in boiling point or freezing point. Even very dilute solutions of high-molecular-mass compounds produce detectable osmotic pressure.
Step 2: Room temperature measurement.
Osmotic pressure is measured at room temperature. Boiling point elevation requires heating and freezing point depression requires cooling, both of which can denature sensitive biomolecules.
Step 3: Molarity, not molality.
Osmotic pressure is based on molarity (moles per litre), which is straightforward to prepare. Boiling point elevation and freezing point depression require molality (moles per kg of solvent).
Step 4: Ideal for macromolecules.
For polymers and proteins with very high molar masses, changes in boiling or freezing point are too small to measure accurately. Osmotic pressure is the preferred colligative property for their molar mass determination because it is measured at room temperature, depends on molarity, and gives large measurable values even for dilute solutions.