I think there are two main reasons.

The first is the small, not standard OLD of the 16" Brompton that hasn't enough width to put a standard cassette. To use a standard derailleur and a standard cassette (9, 10, 11 or 12s), the OLD should be increased to 135mm what Brompton is not willing to do.

The second is the chain tensionning when the rear triangle is folded. The derailleur cage extension arms whatever their design proved not to be reliable on the Birdy 1 and Birdy 2 (the one of Riese & Müller and also lots of different ones from alternative suppliers who tried to solve the problem). The solution came with the Birdy 3 with a totally different concept = a tensionner arm around the chairing.

The 4s derailleur of the P-line and T-line is proprietary with a proprietary mounting on the rear triangle in front of the wheel axle, it only does the change of sprocket, there is second arm placed closer to the rear wheel axle that does the chain tensionning which is very similar to the one of the previous 16: Brompton.