For starters, strongly pre-dates the picatinny rail system.
And while Weaver bases (and several related ones) have existed l long time, there are many others. Lots of euro guns, even military ones, use internal or otherwise obscured attachments, for the reasons
ColtPythonElite stated, about sleek looking and therefore not snaggy.
Another thing many other systems have is that any screws or levers are secondary attachments. They prevent something else from moving and THEN the scope getting loose. Any directly clamped system (Weaver) is directly dependent on the screw threads being properly installed, and not failing in use. Even crossbolt/cross-lug systems (late Weaver, Picatinny) only offload recoil resistance to the crosslug but the scope is going to loose zero instantly if you have a screw/lever failure of any sort.
The DD bases (IIRC) are a system that essentially cannot fail. The scope tube itself keeps them aligned, and by the time you've lost so many screws the scope falls off, base misalignment is not so much a worry
It is overall a pretty slick setup, but is proprietary, intrinsically not QD so cannot take over the whole market, and also people are cheapskates, so not everyone is gonna buy Leupold bases and rings, hence it got pretty popular back in the day, but never dominated.