A central theme in most engineering designs is 'trade off'; you can either go extreme on one feature and neglect the rest, or compromise so as to get something of both. similar concept applies here.
When it comes to electrification, there are pros and cons associated with it. it removes the visual aspect of pollution, and is overall quieter but in turn requires multiple substations, 100's of CKM worth of line to bring power to SS, and then another 100+ Km of 25 Kv line to allow electricity to be used on the line. you need additional staff (and money) to maintain those lines, a sophisticated RCC for its monitoring and setup operations department for managing the locos and staff operating them....
more... these costs are often offset on lines which see heavy traffic, as there the benefits of e-fication outweigh the cons, especially on cost as the fuel cost of electricity is far lesser than that that of diesel. on plains, especially on frequent start stop trains like EMU's, there is an additional advantage which diesel doesn't offer: regeneration. Regeneration on ghats is too risky owing to its loco only nature, but on plains one can liberally use it, and indeed it saves a lot of costs; CR saved 100's of crores owing to regeneration following shift to AC traction for instance.
This particular line doesn't offer those advantages. Being an isolated ghat line, there will always be a limit to how many trains run on it owing to the very nature of railway operation (only one train can be in a block at a time), most trains here are freight which run continuously, and most importantly, no e-loco in India has creep control, which EMD's have, thereby allowing them tremendous TE. On KR, the line actually operates at rather high loads, and is mostly level gradient, obviating the benefits offered by EMD's monster TE, and electrification would save money by means of regeneration. also, the mountainous nature of the line means that a good chunk of line passes through rock cuttings, areas which are clear of trees, so no pruning etc involved.