In general, the political forces that would be arrayed against such a move would be those groups who benefit from the actions taken by each particular bureaucratic agency. They would fear that a president who disliked the mission of the bureaucracy that they support would fire the bureaucrats and appoint ones that were more likely to support his (or someday her) policies.
In our current system, each bureaucracy has its own group that tends to support it. These interest groups are seen as one corner of the “iron triangles” that make bureaucracies so stable. Let us take, for example, the US Department of Agriculture (USDA). It has various interest groups that are very concerned with what it does. These would involve farm groups and also groups representing companies that make farm equipment and supplies. All of these groups have come to have a working relationship with the bureaucrats who staff the USDA.
If Congress were to make bureaucrats easier to fire, these interest groups might feel threatened. They might feel that a president who, for example, wanted to implement strict new rules on pesticide use would come in, fire all the existing USDA staff, and replace them with people who were strongly in favor of restricting pesticides. The interest groups would fear that they would no longer have a good relationship with, or influence over, the new bureaucrats. Essentially every bureaucracy has such interest groups that would feel threatened by the idea of “their” bureaucracy being fired for political reasons and replaced.
One of the primary purposes of civil service is to insulate the workings of government from politics and from instability. There is concern that specialized groups and changing administrations could very well have conflicting agendas. On a more mundane level, in order to run government smoothly, a cadre of civil servants is necessary, so that every four or eight years, there is not a clean sweep of employees, which would create chaos in every agency and department, having to train new people every time there was a new administration. As it is, there is meant to be core of seasoned professionals, not a revolving door.