Purport

Replying to the question asked about the Sanayasr(renunciation) and Karma Yoga, Bhagvan Shri Krishna in earlier Shloka made it clear that both are the noble paths for humans. However, Karma Yoga is better.

But in this Shloka Bhagvan Shri Krishna describes yet another secret. Bhagvan says any Karma Yogi who performs his actions with devotion towards Bhagvan Shri Krishna and by leaving any attachment with the fruits of actions actually already practice Sanayas. Sanyas, as many people consider as the process of leaving the world, the family, society and duty, is not correct. In fact, a true Sanyas can be performed while performing Karma Yoga with purity and without attachment.

Here somebody may ask, what is the difference between two then? The difference between the two is in the process of performing Sanyas and Karma Yoga. To explain it in simple terms, Sanyas is the process of filtering out that karma which bounds humans to the results. This is known as the Tyaga. The state of Tyaga is what called as Sanyas, this is described in following Shloka

Ch18:Sh2
sri-bhagavan uvaca
kamyanam karmanam nyasam sannyasam kavayo viduh
sarva-karma-phala-tyagam prahus tyagam vicaksanah

The Supreme Lord said, To give up the results of all activities is called renunciation [tyāga] by the wise. And that state is called the renounced order of life [sannyāsa] by great learned men.

While karma yoga is the process of performing his natural duty without attachment. Since performing duty as per his natural inclination is easier, Karma yoga is easier. Sanyas, on the other hand, needs strict discipline, and a level of Knowledge to achieve. Sanyas is a process that is a difficult task. Also since a Karma yogi automatically attains the same state of renunciation, therefore, Karma yoga is preferred.