Why are Letter of Recommendations are so difficult to receive?

Why is it so difficult to get a letter of recommendation? like, a few teachers replied and agreed. Only to stop replying let alone give their recommendation.

Luckily i secured one LOR and im working on securing the second one.

Am i the only one that is having an issue about securing LORs? How many teachers did you guys have to approach to secure two (or three) LORs?