Becoz bivalent or so called tetrad are formed in pachytene and starts to disappear in deplotene.

Understanding Chromatid Structure in Meiosis
In the process of meiosis, the stages of prophase I are critical for understanding chromosomal behavior. Among these stages, the visibility of chromatid structures becomes evident.
Stages of Prophase I
- Leptotene: Chromosomes start to condense and become visible, but no pairing has occurred yet.
- Zygotene: Homologous chromosomes begin to pair, forming synaptonemal complexes.
- Pachytene: This stage is characterized by the complete pairing of homologous chromosomes, known as bivalents. At this point, each chromosome consists of two sister chromatids, leading to four chromatids observed in the bivalent formation.
- Diplotene: The homologous chromosomes begin to separate slightly, although they remain connected at points called chiasmata where crossing over has occurred.
Chromatid Visibility in Pachytene
- During pachytene, the bivalent structure is fully formed, showcasing a clear view of the four chromatids.
- The pairing of homologous chromosomes is complete, allowing the chromatid strands to be distinctly identified.
Why Not Diplotene?
- In diplotene, the chromatids are still present, but the homologous chromosomes are starting to separate, making the bivalent less distinct. The focus is on the chiasmata rather than the complete bivalent structure.
Conclusion
The longitudinal duality of homologous chromosomes, evident as four chromatids in a bivalent, occurs in pachytene. This stage is crucial for genetic recombination, setting the foundation for genetic diversity.