This question is related to my finding that Ubuntu and its derivative uses both SYSLINUX and GRUB bootloaders on ISO images for Desktop releases.
When booting from 32-bit ISO image, it is said that SYSLINUX handles the boot process (the boot which shows beautiful splash screen, with progress dot animation).
But when booting from 64-bit ISO image, GRUB is used instead (the black and white screen, clearly showing GNU GRUB version...
on top of screen).
Hence, to rephrase my question, what are the differences which made Ubuntu to use GRUB and SYSLINUX on its ISO images? Why not just use either one instead?
For clarification
This question has been somehow confusing due to my lack of clarification and hardware availability. I have added this answer (or scroll down) to explain better on 64-bit ISO image.
What answer is not
The comment thread below already gave some idea, but I ought to make clear here. The answer is not because either one supports EFI properly. This question aimed to find out more toward fundamental reasons, rather than just EFI support.
Say, if this were to be asked in other way i.e. "Why both GRUB and SYSLINUX are used in the ISO images?" then this might give answer like, "GRUB is included to support EFI-capable systems and SYSLINUX has been always included and works on BIOS-only systems" — which is not my intention.
I must admit that the EFI support is likely part of the answer.
However, I feel that it should not be the only thing in the answer. There must be more than just EFI support, which made Ubuntu to include two bootloaders in their ISO images, yes? Or, is it really the EFI support be the only difference? Help me answer this, if any.
Best Answer
This is my final answer, which is based on information found by matching keyword within 2000+ pages in this listing on Ubuntu Wiki. What I found were dated notes of Ubuntu development and specifications (read: words, words, words), so that took me some time to reach this answer.
Ops, wrong naming
To begin with, the naming of boot loaders shall be clarified:
Name with all letters capitalized refers to the boot loader (e.g. GRUB, SYSLINUX)
Name with initial letter capitalized refers to the project name or, several or all variants of the boot loader family (e.g. Syslinux)
In particular, 'Syslinux' is a collection of boot loaders that includes 'SYSLINUX', 'ISOLINUX', 'EXTLINUX' and 'PXELINUX'
Following the naming convention, the question is actually referring to "ISOLINUX" for "El Torito no-emulation" bootloader, not "SYSLINUX". Perhaps the latter is used interchangeably with the former in old days. Nevermind, then.
Brief history
2005: ISOLINUX is choosen for Ubuntu CD boot loader, instead of GRUB.
2006: gfxboot has been added; This supports information quoted in 2010.
2009: ISOLINUX (noted as SYSLINUX) is still used for booting Ubuntu CD.
2010: ISOLINUX has been used, but GRUB 2 is needed for UEFI support.
Differences found or not
Following brief history, we now understand that:
ISOLINUX was preferred due to GRUB had regressions back then (2005)
ISOLINUX was still preferred despite lack of support for starting the kernel in graphics mode that cause flickering during boot transition (2009)
ISOLINUX has been used with gfxboot to provide graphical menu, which was not implemented or not possible with GRUB back then (2010)
GRUB has been added later to boot with UEFI support since Maverick (post-2010)
Then, I realized that it is not the difference between GRUB and SYSLINUX that made Ubuntu live CD include two boot loaders.
Fundamental reasons
From my reading, these supporting facts actually hinted that:
Ubuntu live CD has been using particular boot loader that had better support for providing graphical menu and theme, and smooth transition to show boot splash. In this case, SYSLINUX (precisely ISOLINUX).
When UEFI systems became increasingly common, then only Ubuntu had included GRUB (precisely GRUB 2) in Ubuntu live CD to boot with UEFI support.
Above all, I believe this answers the question that I had for more than a year and this answer has finally put my curiosity to rest.
TL;DR GRUB and ISOLINUX are both used in Ubuntu live CD for exclusive reasons; Both were included in live CD for better boot experience and hardware support.