Test cricket matches are being played since 19th century.
But, Why these test matches are not played under lights(ie, day and night matches).
It will increase the audience for test cricket and also the standard of it.
The question is very subjective. This can be asked about any rule in cricket or any other sport saying why this and not that. I can, however, speculate and attempt an answer.
Though test cricket is being played for more than a century, playing under lights regularly is only a thing of the last decade and a half. Most stadiums in all cricket playing countries started investing in infrastructure for flood light cricket only by the mid-nineties and early noughties.
Flood light cricket has its own share of limitations, so as to speak.
Colour of the ball - The traditional cherry or the red leather ball is harder to pick under lights. Therefore, there was a need to have a different coloured ball which was easier for the batsman to pick. This gave rise to the white ball which was used along with black sight screens instead of the white screens used in test cricket.
Lifetime of the ball - The white ball gets dirty very quickly in comparison to the red one and batsmen find it harder to spot while facing deliveries off faster bowlers. The white ball loses its shine quickly thereby diminishing the main weapon of faster bowlers in test cricket.
- The Swing - The white ball does not swing as much as the red ball when it is new.
- Dew factor - This does not allow spinners to grip the ball under lights late in the evening. As most of the spinners are finger spinners, this would be a major issue especially in the sub continent where spinners are considered rulers.
I can only guess that while people in charge were finding solutions and alternatives for some of the above mentioned problems, we stuck by day cricket. Now that day and night tests have been tested at county level and have been approved by the ICC, we can only wait and watch the results of whether it adds to the charm of test cricket.