The best reason is that Coimbatore has already got good connectivity to Chennai.....and for your information just don't see towns like Kollam/Kottayam just by comparing the size.....thousands of people travel to Chennai from these towns as they are the business centres of Travancore region.....
And if tommorow someone proposes extension of Cheran Express to Mangalore or Trivandrum what would you say........this train is the fastest connectivity for common people in south kerala to Chennai.....Coimbatore already has many trains to Chennai