It's a small aesthetic matter. When hair is let loose, hair will frame the face, which makes the general shape of the head look smoother; rounder. Now taking into account that smoothness tends to be a feminine quality, (by smoothness I mean the smooth transitions of different body parts as opposed to the ruggedness males are normally attributed), we can apply that to hair as well. Straight and wavy hair is preferred among guys. Curly hair is usually only widely accepted only when it's not natural, and frizzy hair is normally looked down on. The reason for this is due to the hair seeming less feminine. It looses the smooth transitions guys look for, if only subconsciously. Now, braids, one could argue, that braids do have a smoothness to them, but it doesn't change the rougher look that it does have. It loses it's feminine quality and appeals less. This is because the braid is essentially in a knot (though not technically).
Now for my own personal opinion. I don't mind certain braids, but in my experience, the less hair used for the braid, and the more amount of braids there are, the less attractive it looks.
In the end, it comes down to preference. What I told you above was, respectively, the general social view, and my opinion. This in no way is applied to every guy as there are many that prefer braids and/or curly/frizzy hair than the generally accepted wavy or straight.