The KC-135 was originally bought in 1954 as an "Interim buy" pending the completion of the tanker competition (the Lockheed CL-131 "won" over the KC-135 in February 1955, but the CL-131 was just a paper design at that point, while the KC-135 had a flying prototype).
As the urgent need was for bomber refueling (due to the quantities of fuel needed), the boom only was fitted to save money.
In the end, the CL-131 was canceled and the KC-135 was ordered in quantity - still with only the boom.
Some back-history:
The old USAF system, in use with bombers before 1950, was the "looped hose" system, where the receiver was ahead of the tanker, paid out a static line to the tanker which was then used to pull the hose forward to the receiver aircraft.
In Feb 1952 the USAF fitted a B-36H with the USAF LH system for tests with a B-47 (as receiver), with the tests being done in May 1952. In January 1953 "a new and vastly improved British made probe and drogue refueling system was installed".
The USAF had, in 1950, begun using the British "probe & drogue" system for fighters, but the probe was on a drop tank, not the fighter itself, and only a few of the USAF's tankers were fitted with "drogue hose" reels.
Also in 1950 the USAF had begun using the "flying boom" to refuel bombers, and since all tankers were part of the Strategic Air Command (along with the bombers), the bomber method was chosen as the sole system to be installed in the new USAF tankers (and thus, in all new USAF aircraft).