It seems Adafruit wants to promote this microcontroller and it has several good reasons for this (in summary, it's like the SAM21 but with "vitamines"). Why not doing the same here in Arduino? Thanks
It didn't exist when Arduino started their SAMD boards line. There's always a new chip coming out. Slapping one on a PCB is the easy part. Software and community support is the hard part. That support is much more desirable to Arduino users over features. So it makes sense to pick a chip and then stick with that chip until it's obviously outdated.
Arduino made an excellent decision to focus only on a few popular AVR chips for years. Look at what happened with the Primo. .org decided to just slap a new chip on a board but didn't bother with the support side of things and you never see anyone using that thing. There is only a limited amount of resources for development. Make the existing products work well or churn out new boards and ignore the problems with the old ones. Take your pick. I think Arduino is making a huge mistake with all these new boards when there are so many previous projects unfinished.
In current conditions of development you are right.
But if Arduino gave possibility to volunteers to adapt other microcontrollers the conditions would be totally different.
On the other hand, the software of samd21 could only be developed with "active" help of Atmel society. A few years before experience of the DUE had been very painful.
Ad was public and first Zero cards mentioned :
"Developed by Atmel
Made by Atmel in Italy ".
Images of Atmel Zero board were available on Arduino website.
This shows that Arduino company does not have means to devellop itself softs for a new microcontroller.
Nothing says if Microchip wants to continue to help for other similar microcontrollers, because it represents cost that must be followed by a return on investment.
Does a market exist for samd51 ?
On the other hand if you have followed the recent announcements it is Microchip who "advised" Arduino to use the new atmega4809.
MPLabX is now available for Atmel product, see https://www.microchip.com/
68tjs:
But if Arduino gave possibility to volunteers to adapt other microcontrollers the conditions would be totally different.
Not only has Arduino given that possibility, they have done a lot of work to make it as easy as possible. In return, volunteers have added support for dozens of microcontrollers (ESP8266, ESP32, STM32, PIC, nRF51, ATtiny, and many other ATmega parts).
The work Adafruit did to add support for SAMD51 is available for everyone to benefit from. They didn't need to get any special permission from Arduino, they just forked Arduino SAMD Boards and made the necessary modifications. Any volunteer is welcome to contribute improvements to that package or work to improve the community support via libraries, example sketches, tutorials, etc.
To give credit where credit is due, much of the work to add "platform.txt" support to Arduino was contributed by Rick Anderson and Mark Sproul, from their work on ChipKit. The Arduino devs did (eventually) accept this huge contribution and it became a centerpiece of the 1.5 beta and was released years later as 1.6.
And regarding far more credit than is due:
68tjs:
Ad was public and first Zero cards mentioned :
"Developed by Atmel
Made by Atmel in Italy ".
Images of Atmel Zero board were available on Arduino website.
Atmel primarily designed the only hardware. I know, because I personally met the engineer at Maker Faire in San Mateo who had done the board layout work. Their answer to software was ASF and (windows only) Atmel Studio.
The commit history on github is pretty clear who developed the core library, and who is continuing to improve it even now (hint: not Microchip, nor anyone from Musto's defunct dot-org).