The main problem is that there are many PDF viewers out there which are simply too dumb to deal with something which is completely part of ISO 32000 (namely forms, and even intelligence using JavaScript). The PDF viewing components of the web browsers, but also Apple's Preview.app are culprits… and then there is the whole zoo of PDF viewers on mobile devices.
Anyway, PDF (as specified in ISO 32000) provides the best possible platform for forms. The problem is that so far, there is no real test tool which helps determining whether the PDF viewer is actually compliant to ISO 32000. And there is no official tool which determines whether a document is compliant to ISO 32000. In the olden days, a compliancy test was to open the document in Acrobat, and if it worked, it was compliant, otherwise not.
Fact is that the big majority of the mobile device PDF viewer, as well as the PDF viewers coming with a web browser would barely pass a complete ISO 32000 compliancy test.
It is therefore best practice to present an intelligent form in a dumbed down version, which allows at least printing out, or maybe even filling out, and then printing. Only if the PDF viewer supports the smart feature (such as adding up correctly, formatting dates correctly, etc.), the full potential of the form gets unlocked. How much can be done in this way depends on the form itself, but also how much of crappy PDF viewers should be supported.
The situation may be comparable to the era when web pages had to do extensive browser sniffing, in order to display properly.
There are alternatives for forms.
Web forms are a possibility, but they are limited to rather simple forms; as soon as things get complex, they break. And they always need a stable and reliable connection to the internet.
Another technology still floating around in some places was InfoPath, which requires Microsoft Word as filler. And there are also Microsoft Word forms; there are a very few companies who are sufficiently proficient with Microsoft Word to create working fillable forms… but Word is still needed as a filler.
XFA has been mentioned, but XFA is even more restricted in viewers (unless it is used the way it was originally intended, by using a server to render the form in the way usable for the viewer.
So, PDF is the most usable platform for forms; and (on Windows and OSX), the Adobe products are the standards against other PDF viewers have to stand to. That's today's reality. There are other PDF viewers out there which can do quite a bit, and which may be sufficient for many applications. For the forms developer, it means "know your users".