It didn't have enough thrust. Small rocket engines are easier to build than large ones, and weigh a lot less. That probe (I assume you mean Chandrayaan 1) had a 440 N main thruster that it used to get to Lunar orbit. It weighed initially about 1350 kg all from wiki. That amounts to an acceleration of approximately 0.3 m/s/s. You need around 4000 m/s for a trans lunar injection using basic Hohmann transfer, although there are far more efficient trajectories called low-energy transfers.
An extra 4000 m/s with that little thruster would require several hours of thrusting time, even with the rocket equation helping you as you lose mass. Hohmann transfers assume that you basically make an instantaneous impulse change at one of your apsides, which is impossible if you are thrusting for that long near Earth. So, multiple burns are required to take full advantage of the efficiencies gained by such a transfer orbit.