Why is the 55 grain sabot loaded 30-06 Accel. muzzle velocity so low at only 4080fps. is it because they do not pack enough powder in them as for usual loadings?I think 5000 or 6000fps. should be feasable!

Simply diminishing returns. Once you get to a point, adding more powder does very little to increase velocity, just more blast and recoil. The bullet-to-powder ratio is just so high that it becomes unreasonably overbore. An example from Saeed's data here is the .223 vs .22-250 vs .22-243 vs .22-6mm. Adding a few grains of powder when moving up from .223 to .22-250 gives sizable gains along the lines of 400 fps. Adding even more powder going up to the .22-243 gives only 150-250 fps more velocity. Adding even MORE powder going up to the .22-6mm gives almost no gain. That and the fact that the round was probably test-fired from a 24" barrel? Who knows, maybe in a 30" barrel it could pick up a few hundred fps?