It "needs" Source because Source is based on id Tech - which coincidentally is what COD engines are based on as well. Since Bethesda doesn't license id Tech to 3rd parties anymore that's as close as they can get to working with a familiar engine.
The game derserves all the accolades, but it should have been rebranded as Titanfall proper and it definitely should not have been in the middle of 2 FPS juggernauts.
EA, you sandwiched TF2 between BF and COD and cannibalized sales for a return you could have gotten more easily if you spaced out the release dates, repsonse?