Makes you think huh? ill fold (Because im not in the mood to argue with you), makes you think that they should of used bedrock as a base as it would of been easyer
You also have to think though, they had to maintain multiple versions of the Legacy Generation on multiple different platforms, each of which used a different structure
( Hence their discontinuation in the first place, maintaining all of those was deemed too straining ). So which do you think sounds preferable to Devs who just want the finished product?
1: Using the Legacy Engine and Functions which were already being developed before Bedrock reached a state where it could be actually usable and / or even wanted on Consoles.
Or 2: Halt development just so they could wait for Bedrock to get up to speed when development for the Legacy versions was already past the point of Bedrock?
Keep in mind that they were also tailoring each and every Legacy Port to fit the CONSOLE in mind, and waiting for Bedrock to reach the same point in the Development cycle, ontop of optimizations and / or changes to fit the style of a Console would have just been more of a pain as opposed to going with using a underway / mostly finished product in the first place.
The only reason the 3DS Version used a port of PE ( Bedrock ) is because at the time is was less demanding and fit the Console's Performance level better. Even at the time of it's release it was deemed inferior to the already existing Legacy Editions.