Offhand I would say it’s because you have elements in your HTML that are larger than the form and after it in your HTML.
As such, they are lying over the top of the form and so you can’t click on it.
The simple solution is to add:
position:relative;
z-index:2;
to your #contactForm
The better solution would be to restructure your HMTL /CSS as it looks like there are a number of ‘elements’ that don’t exist in HMTL like ‘content’, ‘contenthead’ & ‘contenttrailer’.
I could go on…there are many more…but you get the point.
