The simple answer is congestion. During the initial years of TN's run, there were very few trains on this route. So it was possible to have a train running with utmost priority without affecting the other trains much. But that's not the case now. With more than 100 trains passing through the MAS-NGP section alone ( The number keeps increasing as you go towards Delhi), it is no longer feasible to have such special trains now ( in addition to Raj/Duronto trains which already have the highest priority). It's not a matter of LHB coaches or 130 kmph mps, but rather a matter of track capacity, platform availability at major stations and schedule management. If these problems are tackled, then even with current rake ( it's a CBC rake, which is very good by the way) and current MPS also, it is possible to speed it up.
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more... It's not like SR doesn't have any say in IR. Both these trains barely travel for 2 hours in SR. Speed up gradation would require all the other zones to upgrade their tracks for 130 kmph and practically nothing from SR's side. While getting LHB is a good thing, both these trains have good CBC rakes now ( cleanliness may be an issue, but the rake is ok) so , no hurry for that for now.