Why can only females pilot IS? Why is it that females can pilot IS and males cannot? Is there a special reason? Also, why is it that Ichika can pilot IS?
Why only female can pilot IS ?

The reason why only females can activate the IS (and why Ichika
Orimura is the only male who can do the same) is currently unknown.
But it is hinted in the novel that Tabane Shinonono, the creator of
IS, did not deliberately configure IS to be a female-only platform.
She mentions that it was a accidental side-effect, and since she
didn't really care about that factor, she didn't see any reason to
bother with it.

Source
Why Ichika is the only male IS pilot ?

A speculation on why Ichika can pilot IS.
By ShadowFiend1000
This is my theory behind it, I think the reason why Ichika can pilot
IS is due to Tabane pulling strings at the back. One possbility is
that Ichika is not related to Chifuyu, and like Adam Jensen from Deus
Ex Human Revolution along with JC Denton of Deus Ex, is genetically
altered. This truth is possibly known to Chifuyu, Madoka and maybe
Tabane only.
For those who never played the game. In Human Revolution, mechanical
augmentations require the user to use a drug for life to prevent
gilial tissue build up in order to use the augmentation. Adam was
genetically altered (unknown to himself) by White Helix resulting his
body would not form any gilial tissue. This is a secret only David
Serif and his girlfriend Megan knows about. And it plays a big plot in
the game as well.
Do you guys see the link here now? You might ask who altered Ichika?
My guess, is Phantom Task.
If this is true, it might serve as a massive plot point considering
how many leaders of the world would want to bio mod their soldiers to
wear IS.

Source