Why don't landing pages allow for common fonts?

I’m gutted that I’ve got to spend hours replicating a clients sign up questionnaire using clumpy old forms because landing pages won’t allow Helvetica Neue, which is their brand font, and then I’ve got to try and edit the code of the old form to try to add the font according to a blog post I found (nothing to do with infusionsoft support who offered no help at all) - if you can’t match forms to the brand style of a website, that’s a pretty massive ommission.