The strength of an encryption solution is directly tied to the number of possible passwords. A brute force attack simply tries all possible passwords so it will succeed more quickly if the number of distinct passwords is smaller. Case insensitivity means the attacker only has to try lower case passwords since 'EXAMPLE', 'eXamplE' and 'ExAmPlE' would all be seen as the same password as 'example'.
The number of attempts required for a brute force attack is roughly half the total number of possible passwords which is calculated by taking the number of distinct characters to the power of the length of the password. A typical example character set is upper case, lower case, digits and special characters, which might add up to 96 distinct characters. Removing upper case would decrease the number of passwords in this example from 96 to 70, assuming the three other types are all included. So, for an 8 character password this change would weaken the password strength by a factor of approximately 12.5. [That's 96^8/70^8.]
In a situation where you have diminished password complexity, it becomes much more important that you use a long password. It's a subtle point, but the diminished complexity erodes the value of extra length. For example, 96^9/70^9 is about 17.2, so going from 8 character passwords to 9 character passwords is more effective if the passwords are also more complex.