Why is colonel pronounced 'ker-nall'?

The divergence between the spelling and pronunciation of 'colonel' is huge. There are four Differences (in order of appearence):
      1: Trading the 'l' (lowercase L) sound for an 'r' sound.
      2: Trading this newly-made 'or' sound for the 'er' sound.
      3: Removing the second 'o'.
      4: Trading the 'el' sound for the 'all' sound.

Difference One

The route suggested by linguists for Difference One (the 'l' being pronounced as 'r') is historical.
      Colonel comes from Italian 'colonnello'. This refered to leader of column of soldiers.
      In French it changed to 'coronelle'. Here is the origin of the 'r' pronunciation. This was due to dissimilation, whereby one of two same sounds within a word is changed to make pronunciation of the word as a whole easier. 
      English borrowed this French word in spelling and pronunciation. Later, the English shifted the spelling to align with the original Italian spelling (whilst keeping alignment with the French pronunciation).
      So, the 'l/r' Difference isn't due to the English pronunciation changing but instead it's due to the English spelling changing. This puts it at odds with the other Differences.

Difference Two

This Difference has clear, simple analogies in multiple words. Pronouncing 'or' as 'er' thus isn't an exceptional change.
      Such words include 'word', 'worm' and 'attorney'. This matches Difference 2 in 'colonel' (the 'col' being said as 'ker'). 
      This one spelling consistently changes into one corresponding sound. If these changes happen in multiple words, why not also happen in 'colonel'?
So, Difference 2 does happen in many words. But this doesn't answer why it happens. Assimilation seems to be the perfect explanation.
      Different sounds are made in different places in the mouth. It's easier to say sounds when they are close together in the mouth. As such, sometimes people move a sound to facilitate this. It is called assimilation.
      Think how an 'n' is usually said near the front of the mouth, like in 'pan'. But in words like 'pancake', 'ink' and 'incredible', the 'n' is nearer the back, like the 'c/k' that follow it. 
      Additionally, think how 'bless you' is said 'blesh you'. The end of the first word becomes closer to the start of the last word.
      In 'colonel', the 'C' is at the back of the mouth. The tongue doesn't have to move far from this to make the 'or' sound. However, the tongue doesn't have to move at all from the 'C' to say 'er'. No wonder it changed!

Difference Three

Losing the second 'o' isn't that controversial. Letters get dropped from words all the time to make pronunciation easier. The process in Difference Three is called syncope. 
      This is when a sound within the word is lost, particularly unstressed vowels. Think how library is said 'libry' and Gloucester is said 'Gloster'. 
      In 'colonel' it just so happens to be the second 'o' that's unstressed and syncope-d out of there. 

Difference Four

This Difference can also be seen by analogy.
      Think of words like 'camel', 'hazel' and 'towel'. Though spelt with an 'el', they are pronounced like 'all'. 
      Just like in Difference 4 with the ending of 'colonel'. As before in Difference 2, we can see that a spelling reliably changes into a set sound.
So, why does Difference 4 do this? It's all about ease of pronunciation. 
      The 'el' in words were said as a dark L, where the back of the tongue is raises to the roof of the mouth. This is an unusual position for the tongue, particularly being scrunched up so far back. 
      The 'all' sound is near to the dark L, both in the position of the tongue and the sound itself. It's easier and, as seen before, sounds change to the easiest pronunciation available. That, and 'all' is far more common and thus far more comfortable. Understandably, comfort equals ease equals common use. 

Conclusion

The four Differences between the spelling and pronunciation of 'colonel' make pretty much everyone annoyed. But the reasons behind such divergence are fascinating.
      When asking why 'colonel' is pronounced that way, people always focus on Difference One. It's the four Differences together that make 'colonel' so interesting. As such, this singular focus is baffling. 
      That's why I had to fill in the gaps with legitimate linguistic processes like analogies, syncope and assimilation. Understanding ease of pronunciation helped to fill in the gaps. 
      As with any skill, practicing the application of knowledge and techniques provides ever more accurate results. Doing this with the word 'colonel' was an interesting process.
      So, why is 'colonel' said as 'ker-nall'? Because, like all words, pronunciation changes over time and these changes follow particular, predictable processes. (No matter if people wished they didn't.)