thomasfo
There's a thread specifically on the issue of air data sensing
here in the Tech Log forum with some info on this issue.
One issue with a dissimilar design is that it becomes quite hard to ensure similar performance from dissimilar designs, which makes it more difficult to monitor for errors between different types of sensor, as the "error" may not be due to a failure but simply the expected issue. Since errors between different air data systems can be quite important for things like RVSM performance, this isn't a trivial issue.
You also have to remember that all probes,even if differently sourced, would have to be to a common specification. If the aircraft then encounters conditions outside those covered by the spec, even dissimilar probes could fail together.