Actually, a roughly 2-day rendezvous is standard*, and the 4-orbit expedited rendezvous, while being practiced more in the recent years, also involves phasing the International Space Station's orbit so it precisely coincides with the launch site at precise time (thus the instantaneous launch window), which sometimes isn't possible.
This Soyuz TMA-18M rendezvous was one of those latter cases, because the station had to do Pre-determined Debris Avoidance Maneuvers in July 2015, and also slight phasing corrections for the next week's Soyuz TMA-16M landing, which set it out of phase for the expedited rendezvous with Soyuz TMA-18M. Since it was already out of phase, it also performed an orbital reboost on August 31, just two days before the TMA-18M launch. An additional reboost will be performed on September 7, to finalize setup for TMA-16M landing and next uncrewed Progress 61P launch to the station on October 1.
*I said in the beginning that the 2-day rendezvous is standard, so I should also mention that failing to perform expedited rendezvous for any reason, one of the fallback procedures is the switch to 2-day rendezvous. So 2-day rendezvous mode is always an option with crewed Soyuz launches, while the expedited rendezvous not necessarily. Thus standard.
The fast approach mission is relatively rare, and has only been done recently. According to this news article:
Soyuz TMA-18M was to be tasked with achieving a fast-track, six-hour rendezvous with the ISS. However, that approach was cancelled due to the current altitude of the Station.
To achieve such a fast-track docking, Soyuz TMA-18M would have had to perform the first two pre-programmed engine burns during its very first orbit of Earth.