Why was Pururava not purified by worship of Lord Vishnu, as was Dhruva Maharaj?

From: Chethan

In SB we see pururava praying to Lord Vishnu for getting Urvashi and Dhruva approached Vishnu to get a great kingdom. Dhruva got purified, but pururava got Urvashi and later got frustrated.. He didnt get purified while following worship.

Guru is not there in case of Pururava whereas in Dhruva`s case, Narada muni had given instructions to follow.

Even then, we tell that whoever approaches Krishna/Vishnu for any material desire, get purified of their desire..(akama sarva kamova…) why is it that Pururava though saw Vishnu in person , still had lust in his heart for a mundane woman? please explain