What valid reason can I give for my low CGPA to a prospective PhD advisor?
Honestly I don't have any valid reason, I did not pay attention earlier to my academic studies.  But I think I can't mention that.  What can be some possible reasons that I can tell him?

On the contrary, if you think that was the cause, then that's exactly what you should say.  Prospective supervisors are interested in a lot of aspects of candidates, and one of the important aspects we are interested in is the capacity for students to objectively review their present deficiencies, or deficiencies in their past work.  It is possible to assist a candidate with lower level grades to improve their work, but it is much harder to deal with a student who is not honest about their own work, or unwilling to be honest with their supervisor.  Thus, demonstrating the capacity for self-reflection and self-criticism may be viewed positively, even if it involves admitting you slacked off.  (Academics are humans too --- we know that there are other fun things in life for young people to do besides studying hard.)
Setting aside whether that will assist your application or not, you need to grow some character --- start to make a habit of being candid with people about your actions regardless of whether this will advantage you personally.  Deficiencies in your work can be fixed over time, but getting into the habit of searching for false excuses is a deficiency that will damage a lot more than a PhD application.
Reframing your question, there is no &quot;valid&quot; reason or &quot;invalid&quot; reason for your lower grades --- there is just the truth, and not the truth.