Facts based explanation
According to traditional superstition, such clovers bring good luck
though it is not clear when or how that superstition got started. The earliest mention of “Fower-leafed or purple grass” is from 1640 and simply says that it was kept in gardens because it was “good for the purples in children or others”.
A description from 1869 says that four-leaf clovers were “gathered at night-time during the full moon by sorceresses, who mixed it with vervain and other ingredients, while young girls in search of a token of perfect happiness made quest of the plant by day”. The first reference to luck might be from an 11-year-old girl, who wrote in an 1877 letter to St. Nicholas Magazine, “Did the fairies ever whisper in your ear, that a four-leaf clover brought good luck to the finder?”.
According to me
The same reason why people say “their wishes come true when wished upon a shooting star”
Reason- it some legend that people have heard of, doesn’t matter whether it is true or not. It won’t matter either way whether we believe it or not, so why not think positivity and believe it true.
so basically 4 leaf clover is considered lucky because people believe it lucky. There is simply no proper reason.