The Eastern Range run by the Air Force has a 2 or 3 week maintenance window. Thus no one is launching from either Cape Canaveral (Where LC-39A is located), nor the CCAFS (Cape Caneveral Air Force Station, where LC-40 is located) locations.
SpaceX is using the time the range is down to modify the RSS (Rotating Support Structure) that is left over from Shuttle days, to remove more of it, and to work towards installing the Crew Access Arm, needed for manned Dragon V2 flights.
SpaceX has two (at least) major modifications on the books for LC-39A. Install the Crew Access Arm, and add the Falcon Heavy hold downs and equipment. It is unclear how much of the work they will get finished in this window.
They have said the Falcon Heavy modifications with take 60 days, and will start once LC-40 is reactivated and flying. I.e. They need to keep a pad open and launching.
That is why they were in a hurry to get the Intelsat 35e launch off before the range shut down.
Then once the range is ready again, they have to find a launch time, and the CRS-12 mission, which is next, has ISS requirements for launch dates. The ISS has a lot of Visiting Vehicles (Soyuz, Progress, Dragon, Cygnus, and soon CST-100 and Dragon V2) and scheduling crew time to unload and reload is a complex ballet. Thus the date of CRS-12 is set by the ISS schedule and there may not have been time to slide another launch in beforehand.
Here is a mostly information null article on this very issue.