From their perspective, because they can't imagine that Time travel is possible...
The Reverse Flash has had a yellow version of Barry's Suit for at least 14 years.
Sisco's current age minus 14 = (give or take) 9 years old.
1. Sisco is Vandal savage. He's immortal and ####ing with every one. He's like Asimov's Mule. The last bastard you would suspect of being a master mind super villain.
2. Sisco "borrowed" the suit design from someone/where else. Maybe it was a deep sea diving suit like the Flash's costume in the 90s series. Are we sure that the design was specifically for the Flash and not reappropriated tech?
3. Sisco said that there were 3 suits in the episode that plastique destroyed one. Sisco may be able to design this suit, but he probably has to send away to 15 different factories to make and assemble the components. There's could be dozens of Flash suits across the world in various stages of construction.
4. Are there really only two people working there at Star Labs? Sisco and Caitlin. Sisco and Caitlin should be division heads with 80 people under them each. Is there a hundred and 60 people in another room answering to the Wonder Twins? Unless before Star Labs shit the bed, they were unpaid interns, and are still unpaid interns.
5. Did Multiplex really die? His power isn't exactly cloning since he seems to replicate the clothing he is wearing and maybe anything in his pockets (including money)... Was he replicating the guns in his hand every time he copied himself? If Multiplex is alive (In a cell under Star Labs) or they have figured out how to replicate his replication powers, Sisko/Wells could have built even "hundreds" of Flash suits at zero financial and resource cost. But surely dead is dead?
6. Sisco was given the Flash suits by the Reverse Flash and told to follow orders years before Sisco met Barry.
7. Sisco is the Reverse Flash.
8. The Reverse Flash has access to the Cities CCTV network and has been making sketches of the suit while erasing all recorded data of his adventures that the Task force might want to take advantage of...
9. The Reverse Flash is the janitor working at Star Labs and he borrows a Flash suit when the others are... A super hero can too be a janitor! Hong Kong ####ing Phooey! Oh, and Superboy for most of 2003. DC Comics thought it was funny because Superboy was a Super(intendant)... No, Hawkeye is not a Janitor/Super because although he is forced to plunge other people's toilets, he also owns the building he is doing this handywork in.
10. The US Government. It's a quick turn around but either Wade Eiling (The Army) or Amanda Waller (Argus) after seeing the Flash are going to want a response/solution to the Flash, and they will throw billions of dollars at batteries of scientists until they have one. They could track down the Reverse Flash who has been operating for the last fourteen years and then outfit him with kit they built after seeing Barry in action.