After the AMOS-6 explosion on the pad at SLC-40 in September 2015, SpaceX spent a lot of time rebuilding SLC-40, including a very big update on the TEL (Transporter/Erector/Launcher). Whenever they speak of it (Gwynne Shotwell or on the CRS-13 launch webcast) they mention that the original TEL was built before they ever launched a Falcon 9, and they have since built Vandenberg's SLC-4E, and LC-39A.
Thus they have a lot of experience now on how a TEL should be built.
When they revealed the LC-39A TEL it was very different from the previous looks and was mostly enclosed. But on the CRS-13 launch it because clear that the newest TEL was still a lattice design.
Initially I was wondering why they did not enclose it, but perhaps a better question might be, why was the LC-39A TEL enclosed?