Because the idea of a collective defense treaty of all of the USSR's allies was Polish although the actual form of it would be drafted by the Soviets.

On the 'European' (supposed to be pan-European but only the USSR and its allies actually attended) Defense Conference in Moscow in November 1954, called in response to the upcoming admittance of West Germany into NATO, two proposals for a collective military pact were raised:

A) The East German (DDR) one that was also supported by Czechoslovakia that that sought to create a tripartite alliance between the two countries and Poland. This proposal would maximize the role of the DDR's military hence why it was seen by them as preferable; the three countries were also the most endangered by the entry of West Germany into NATO while it was not that much of a physical threat for others.

B) The Polish one of a collective security agreement between all of the USSR's allies centered on the Soviet Union. In contrast to the DDR's proposal this would integrate its military more tightly into the bloc and make the Polish army the second largest after USSR in this arrangement.

In February 1955 the USSR would finally choose the Polish project with Warsaw as the place the treaty would be signed in.

Sources:

MASTNY, Vojtech: The Soviet Union and the Origins of the Warsaw Pact in 1955

CRUMP, Laurien: The Warsaw Pact Reconsidered: International Relations in Eastern Europe, 1955-1969; 2015