If you’re getting 5.5% with a cash like instrument you are not going to want the long duration treasury at 4% recognizing that the treasury reduces sales of longer duration and feeds the short duration need. If treasury was to sell more longer duration the market would want a higher rate which the fed / treasury does not want right now. When they reduce front end rates then the long end will be more valuable. Unless inflation picks up. Then if treasury want to go out into the direction curve the market may demand higher rates. The treasury again has a choice pump more short term tbills until someone comes to the table who is willing to own longer duration. There has to be some incentive to do so (higher rates is usually the incentive for holding duration).