Few things to note here.
1) Loco link is not decided based on HP of the loco.For most of the trains for IR (other than those which have very aggressive schedules or high speed runs), the loco link is mainly decided based on loco availability and utilization. Locomotives 'cannot' be kept idle. They have to be utilized to the maximum possible extent.
For example, the Bangalore-Bangarpet express used to run with a WAP7 all the time. This was a short distance, slow train with a short rake, and yet...
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it got a WAP7. This was because, the WAP7 leading the train arrived in the evening at SBC and would have to remain idle till the next morning. So instead of keeping it idle, they use it for another train service. Diesel or electricity is not the only thing the railways has to pay for. Even if a loco is kept idle, they are indirectly losing revenue.
2) This is a more important factor. There is something called an optimal 'HP/ton ratio'. Freight trains in the USA typically maintain 2HP/ton. That means, for a freight rake weighing 1000 tons, the locomotive(s) leading should have at least 2000HP. This ensures the best balance of running time, energy costs etc.
For a passenger service, the HP/ton has to be much higher as passenger trains need to run fast and stick to a schedule. That way, for a 16 coach passenger, which weighs about 800 tons, a WDP4D loco will give about 5.5HP/ton, which is a very good ratio for a passenger train (much better than that of an ALCO, which will only be 4HP/ton).
Yes, an ALCO may be 'enough' for this load, but it is not the best possible loco for this load.