It is relative common for software to need integers larger than 53 bits. It seems quite uncommon for software to require Decimal arithmetic. Do you have a motivating example?

I actually didn’t look hard enough: there is a proposal[0] for something that could be called ‘BigDecimal’ in ECMAScript, but it’s a stage 0 proposal[1]. As for why a decimal type hasn’t progressed further in ECMAScript you’ll want to read the notes [2] from the meeting where the latest proposal was discussed.

[0] https://docs.google.com/presentation/d/1jPsw7EGsS6BW59_BDRu9...

[1] https://github.com/tc39/proposals/blob/master/stage-0-propos...

[2] https://github.com/rwaldron/tc39-notes/blob/b8da60318b564f13...