'Why?' is a difficult question in grammar. It's not like some great designer created the language out of thin error, planning and constructing things to be as logical as possible.
Some grammar rules apply everywhere, and some rules have exceptions, and those exceptions have exceptions, except when people feel like saying something else.
The rule for comparatives in English is to say 'more X'.
The exception is, if X is short, then say 'X-er'. Usually 'short' means one syllable.
But short is not always obvious. If this shirt is red, but that one has more in it, then it is redder. If this one is purple the other one is more purple. But if this one is yellow... officially the other one is more yellow, but informally people will often say 'yellower'.
Why is there this rule? Language doesn't follow logic strictly (unless it feels like it). One can give a history of the effect, one can give logical justifications why one is easier to understand, (I personally think 'bigger' sounds better and less blunt than 'more big', but then I would because it is so natural to me) but really in the end it is often just fashion that governs these rules.
In short, that's just the way it is
(in other Germanic languages that make the comparative the same way, the rule might be different: in German, you only add '-er' for example 'intelligenter' which sounds funny in English, but surely 'more intelligent' sounds funny to Germans).