Apart from being a regional variant, here's the physiological reason for chimley::
" “m” and “n” are both “nasal consonants.” It can be difficult to physically put two of the same type of sound together (e. g., “anemone”). “L” is a “liquid consonant, “ and… here's where I ask you to make a sound, out loud, please do it …make an L sound, like, start to say “like,” but only say the L part, and keeping your mouth and tongue in that very same configuration, say, “no.” Did you notice how easy that was? L has much of the same physical action as N, but the L is easier to form, and it's more “sonorous” (another technical term), so it's easier to say after the m and before the vowel. say “imly” and “imny,” in a loop, one after the other, over and over a few times, and feel the physical stresses involved, subtle tho’ they may be."
"In the situation of the written word, chimney, the final syllable begins with the voiced, sonorous, continuant, alveolar, nasal consonant, N, which, without affecting the intended meaning, may be substituted with the voiced, more sonorous, continuant, alveolar, lateral approximate consonant, L."
Courtesy of a Quora contributor