Because that is what the whole Ranma 1/2 story is Ranma and Akane.
Almost every element indroduced in the manga is for the purpose of causing
conflict of a comedic nature between them.

I only have the vis collections but is there a single story where Akane is
not a principle character.
 
Even in the breaking point storyline where Akane could have been left home.
She was included and her cooking was indroduced as another point of
contention between Ranma and her.

All the other Fiancees are just there as obstacles as are there nosy
families that can't let them have a minute of privacy.

What happens when ever they are about to have a tender moment interuptions.
 
However in the manga there is just no leeway allowed as to anyone but Akane
getting Ranma.

Any other romantic partner is only introduced so that they can choose each
other over that person. After all how could we know how deep there love is
if they had no other options.

Having said all that I see no problem with a fan fiction looking at an
alternate pairing thats what fan fiction is for. If however you are going to
write something that has any substance and is not a ball of fluff. You are
going to have to make the reasons for why her and not Akane a big part of
the story.

  As for A long And Winding Road I think I will wait and see where it goes.
After all this is just chapter one. But Zen there seem to be a very large
number of people out there sharpening, loading and charging various
implements that could be hazardous to your health. So keep your head down as
you type.

Shing---Shing---Shing---Shing---Shing---Shing---Shing---Shing---Shing---Shing

Lets see now --Damm cut my thumb. Got to go get a bandage see you later.