Because by the second day of incubation any cells that have undergone reversion mutations give rise to revertant colonies like rats leaving a sinking ship.
13 seconds sound clip from the Blade Runner (1982) movie soundboard.
You can hear this line at 01:24:05 in the Blu-ray version of the movie.
Quote context
[...]
- To make an alteration in the evolvement of an organic life system is fatal.
- A coding sequence cannot be revised once it's been established.
- What about EMS recombination?
- Ethyl methane sulfonate is an alkylating agent and a potent mutagen.
- It created a virus so lethal the subject was dead before he left the table.
[...]