Companies are extremely cautious to embrace new technology, even if the technology is not so new. One such very common issue of this is the development of commercial applications for Linux.
Eclipse has long been an expandable development platform for multiple operating systems. As such, it's the perfect platform to develop a cross-platform commercial IDE on. Zend ported their IDE over to it using mainly PDT and other various OSS tools, and Adobe is utilizing it for their Flex language (the logic in flash player). Despite this, they do not fully support Linux as a full/stable release.
If you want to vote for the Linux version of flex builder to be produced in full, please take a second to head over to http://bugs.adobe.com/jira/browse/FB-19053 and vote for it.