Couldn't they switch on more lights? What is the bad light rule for limited overs cricket?
Yes they CAN. But only if it is available for that particular match. Remember, the issue is for day matches only.
Umpire's role on bad lights. From Standard ODI match playing conditions PDF: (emphasis mine)
3.7 Use of artificial lights
If in the opinion of the umpires, natural light is deteriorating to an unfit level, they shall authorize the ground authorities to use the available artificial lighting so that the match can continue in acceptable conditions. In the event of power failure or lights malfunction, the provisions relating to the delay or interruption of play due to bad weather or light shall apply.
So the umpires can use floodlights in day matches when they think the natural light is deteriorated to an unfit level. But that is possible only if it is available for match. Floodlights are not compulsory for the day matches. And on the match between Sri Lanka and New Zealand the use of floodlights was not permitted by SLC due to financial savings which hurt their own team. That's why they didn't use floodlights.