As a developer I can tell you that to re-write an entire application in another language and ensure it is stable and secure would be a major undertaking (ie hellishly expensive). Also the gui components would need to be re-written to not use dotnet / aspx or other Microsoft technologies. The switch in languages to do that would involve a whole new developer team with skills in languages such as PHP, Python or other non-MS languages. That would also double the support requirements and costs with two teams - one for each stream.
SmarterMail is written specifically as a replacement to a Microsoft Technology - ie Exchange. It makes sense to keep it purely in the MS camp.
To keep other technologies compliant when you swap to linux would also be difficult. Active Directory is one that springs to mind immediately. The API and specs are not always open or easy to comply with. iMap is one causing huge issues with even larger dev groups such as Google having open tickets with MS for two or more years in regards to iMap issues with outlook and standard iMap resources.
There are two other projects that allow outlook Native connections (please excuse me using competitor names in this thread) ZenTyal and Zimbra. both have issues integrating into an MS environment. I run servers using both those technologies and the pain involved in getting them working has been immense. And before anyone says that's cause I'm an MS man, I've worked with Linux since Caldera RC 0.6.? and have run commercial Linux Servers for over 15 years. I'm also a Lotus Domino expert and have worked with over 12 different types of commercial mail server in my time ( not including counting different versions of Exchange).
It is very unlikley any change would be made by SmarterTools (and I cant speak for them but only in my own opinion) purely to make it Linux compliant. Linux already has well tested POP, SMTP, IMAP, Spam removal etc stacks. From a business point of view Smartertools is not likely to compete in that market.
My 2.5c worth :)