So, why exactly is the tesla cannon in the mojave if the BoS just made it in DC

Because the Enclave had it way before the BoS Had there hands on it.

Check the B-29 Crash sight... They had a prototype that was working very effectively long before the BoS pushed east.

The BOS got to DC quite a while before the start of FO3, and FO3 takes place several years before FONV. The more easily attainable Tesla Cannon in FONV is also a prototype.

Well given that the Tesla Cannon is mentioned on the front of the Energy Weapons magazine not only was the Tesla Cannon a pre-war weapon but also one that was apparently public knowledge, i.e., not classified infomation.