Indexing probably had something to do with it as well.
You need a stiff, incompressible outer, that's as short as possible, couldn't easily do that with bar end shifters, especially not with the outers that were generally around when early indexed shifters started to be available.
Once we got into the proper incompressible outers (the SIS/SP40 type stuff) and light action rear mechs it got much easier to index with a longer run of outer. But by then, we had STI.
(and regarding the earlier poster commenting on reliability, STI/Ergo are pretty damn reliable these days. Yes there is lots to go wrong, but it very very rarely does.)