Maybe the IMF does not want El Salvador to be an example that every country will follow because it will clearly make any country able to quickly pay off the loans that the IMF provides and is considered the profit that comes from the bubble. From here we already understand what the IMF task is like and what is clear is that they are worried if the influence or example of El Salvador becomes a new benchmark in the global financial industry.
Anyone who has some knowledge of IMF works understands that the interest rates that they charge are far lower than the global benchmark across the financial institutions. IMF doesn't think that El Salvador can pull themselves out of poverty by using Bitcoin, or similar investments. They are meant for the development of their country, and purchasing Bitcoins doesn't help with that at all.
IMF would love to have countries to pay off their loans ASAP, because both sides stands to benefit. They aren't loan sharks and this is a conspiracy that is clearly disproven.