The basic reason the coup failed was its premature birth. How? First, the coup was planned for a later date but was moved up. Then the rescheduled coup, which was to have started at 3 a.m. July 16, was moved up yet again by five hours, to around 10 p.m. July 15, after the coup attempt was discovered.
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With the indictments in last week's Izmir espionage case, the Turkish judiciary was gearing up to target about 1,000 Gulenists in the TSK prior to the supreme council meeting. With so many detentions just before the meeting, the government was intending to compel the TSK command to carry out mass purges.

When the coup planners learned of that intention, they moved up the date to precede the council meeting. This turned out to be a suicidal move.
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Al-Monitor's sources said coup leaders had planned to move at 3 a.m. to take over by 5 a.m., when people would be asleep and streets would be empty. But this didn't work. Around 4 p.m. July 15, 11 hours before the coup was to start, the national intelligence service MIT learned from radio and telephone intercepts of TSK personnel known to be Gulenists that some units were gearing up. MIT informed the chief of general staff at 5 p.m. At a meeting at the chief of general staff headquarters, officials decided to close Turkish air space to all flights as of 6 p.m. and bar military units from leaving their barracks. While that order was being written, Land Forces Commander Gen. Salih Zeki Colak was called to the general staff headquarters.

By 7 p.m., coup plotters realized that their plan was exposed and were able to take Akar hostage. ... At the end, the plotters couldn't get Akar's signature, so they sent out an order signed by a brigadier general and launched their coup around 9:30 p.m. — that is, 5½ hours before the planned time. The first indication of the rash, reckless actions by the Istanbul wing of the coup was sending out small units commanded by junior ranks to streets on the pretense of exercising or fighting terror.