Because Macs have been unix-based for a while (originally modified BSD, IIRC), 9/10ths of the work involved in any port to Linux is probably done.
But I'm sure Trimble do look at it from time to time.
That said, other people are making money from Linux: I have used AfterShot for photo processing workflow for years (Corel now own it - used to be called "Bibble"). It has an enthusiastic user base, and I pay for upgrades regularly. But the whole pricing model is quite different to Trimble's for SU: the full package costs a lot less, so is an affordable step-up for far more people. Similarly I use paid-for specialist panorama processing software from a small company: binaries for Win, Mac and Linux, and it's fine.
There is a sweet spot for pricing that maximises revenue (basic marketing/economics). IMHO, Trimble are way off that - the licensed version offers too little added value for too much cost - the step-up is simply too big. They would also do better with three tiers: Freeware, Licensed (full features, no support) and Support Contract (for professional users and larger organisations, on a per-seat basis). Tier #2 is an almost-no-cost revenue stream, but from it you can also get relatively inexpensive product marketing feedback, early warnings of security issues, etc. and a strong community (yes I know that already exists, but the signal/noise ratio would be better).
There's no reason Linux would necessarily add a penny to their post-sales support costs directly (apart from maintaining the code, that is).