Why does the shuttle throttle on ascent?

Henry Spencer wrote:
Three specific snags also aggravated this problem on the SSME:

[...]

In his book "Advanced Engine Development at Pratt & Whitney," Dick
Mulready devoted a chapter to the competition to develop the space
shuttle engine. By the time of selection, P&W's XLR129 had over 251
seconds of operation, versus 0.461 for Rocketdyne's engine.

During a visit, Dick Bissell, a consultant for United Aircraft and
formerly of the CIA and progenitor of the U-2 and Blackbird, said,
"I am sorry, but you cannot win. It was already decided in advance.
The only reason for the competition was to transfer your technolody
to them."

Does anyone have any opinion on the relative merits of the P&W and
Rocketdyne designs? On the politics?

Thanks,

Paul