If I were to hazard a guess it would be because in my experience, patent is way more technical. TM isn't (done work in both). I personally find TM a lot easier to grasp, and a lot of patent lawyers also have a science background. Another thing I was discussing with a senior the other day was that patent lawyers are generally fewer, but that's just speculation.
In general a patent associate can bill more and hence be more "productive" as fees for any kind of patent work is higher. Patents team is largely based out of Mumbai - it is run from Mumbai where there are few firms with large patent practices, as compared to Delhi where there are several. Hence, unlike in Delhi, there are fewer opportunities to explore and jump.
If so, why are they discriminating against TM associates? Surely both TM and patent associates are putting in the same hours!