No particular reason. Rachel’s just more popular than Leah, so one hears more about her.
Leah from the Bible is regarded as a saint, and obviously she is one of the matriarchs of Israel. She is mentioned in the Book of Ruth in a wedding blessing, which names both her and Rachel as women who built up Israel. Jewish tradition regards Leah (as well as Rachel) as having been a prophetess, because the names she gave her children were prophetic and were revealed to her by God.
In medieval times, she and Rachel were regarded as an OT version of Ss. Martha and Mary. Leah and Martha represented the “active life,” while Rachel and Mary of Bethany represented the “contemplative life.”
If you want to read about Leah in older sources, you will find her name being spelled Lia (the spelling in Latin Bibles). Her name means “cow” in Hebrew (possibly descended from a word for “antelope”), just as “Rachel” means “ewe.” She was buried in Machpelah, the same place as Abraham and Sarah (and other patriarchs and matriarchs).
It looks like there was no definite day for St. Rachel or St. Leah that was in force everywhere. Some places went with the 30th of September (“pridie Kalendis Octobris” is the old Latin way to write the date – it just means “the day before the first day of October”) for St. Rachel. Others went with November 1 (Kalendis Novembris) for both Leah and Rachel. (Nov. 1 is All Saints’ Day, so obviously it is every saint’s day!)
(Just FYI, St. Abraham was on October 9, and St. Sarah was usually celebrated on the same day, or on August 19th. I can’t find St. Jacob’s Day off the top of my head, but he had one too.)