Basically because they truly knew the strategic situation and because of politics.
The "Take Berlin and Win the War" stretegy ws purely political symbolism. The Western Allies could win the war without taking Berlin, and certainly had an inkling of the enormous casualties this would entail (some post-war estimates count the Russians as having lost 300,000 men in taking Berlin ALONE). It just would have required vast investments in time, blood and iron that would have been hard to take.
The reality was that Britain was exhausted and out of soldiers. France barely existed as a nation, let alone one capable of fielding a massive army. The only available (potential) ally was Germany, and they had just lost 8 milllion or so people, and the alliance would have been politically unpalatable.
It was best to let the Russians and Germans fight for Berlin, and bleed the hell out of each other while they did it. For all practical purposes, the Russians beat the Germans in any case, with the US/Anglo-Fench alliance more or less nibbling at Germany's perimeter from 1941 until Normandy. That tells you something about the ability of the Western Allies to actually carry the war to Germany, let alone Russia, prior to 1944, and even then, we suffered horrendous losses.
Hell, we wasted near on 60,000 men on "strategic bombing" campaigns that were nothing of the sort and did very little to actually stop German war production. How long could we sustain that kind of effort?
Think of it this way: from the Normandy beaches to the Rhine is a relatively short distance (if memory serves, more or less 500 miles, give or take). How long did it take to actually reach the Rhine and what difficulties did the allies face in doing it? And how close did the whole thing come to disaster after the fighting in the Hurtgenwald and the Bulge? And against an enemy that was increasingly running out of manpower, resources and was fighting on two fronts?
The very effective German soldier was even more so on the defensive than he was on the offensive. Germany fought a defensive battle all the way across France and across trhe Rhine, and made the Allies invest a year in the process, with attendant loss of life and limb.
The Russians truly carried the war to Germany in a way the Western Allies could not.
"The Russians truly carried the war to Germany in a way the Western Allies could not."
That sounds straight out of Pravda.
How many weeks was Patton's army ordered to sit on their haunches so that the "Russians" could take Berlin.
Ah - incidentally, it was not Russians but the First Ukrainian Front that was thrown into the breech by Stalin (as they were in Stalingrad too).
Get your facts straight. The Poles and Ukrainians would have paved the way to Moscow for Pattons Army.