There are a few reasons. Firstly high speed trains run on new expensive railway lines. The cost of the smooth rails and the advanced signalling system required is so great that adding the catinaries and transformers to allow electric trains is a minor cost and well worth the expense.
But in addition to this a big issue with high speed rail is the time it takes to accelerate up to those speeds. There is not much time accelerating from one station up to high speed before you need to slow down for the next stop when going at these high speeds. In addition to stations there are also corners with slower speeds and other obstacles which demand lower speeds. So while weight is generally not an issue for trains that go slow and steady for hours between stops it is a big deal for passenger traffic, especially high speed trains. The diesel engine in the locomotive is basically too heavy for the high speed train. It will add a lot of distance and time for both accelerating and slowing down because it is so heavy.
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