The answer you are looking for is in the 372th issue of the Dragon magazine, namely the article "The Gnome, The Bad and The Ugly."
It is an article by James Wyatt, where he explains like this:
First he points out how cute and adorable gnomes are and the issues related to creating "badass gnome" as he had troubles playing such gnome in DnD and WoW respectively.
Basically, I was having fun playing up the comical aspects of the
gnome race in WoW, and I had a sudden insight into
the lasting appeal of the gnome race that had escaped
me for so long.
Gnomes are cute, and gnomes are funny, and
sometimes that’s okay.
He expands this point right after by describing the creation process of gnome race for 4th edition.
In late 2006, the world design team for 4th Edition
discussed gnomes and came up with a story we were
pretty enamored with. It gave them a dark past as
ally-slaves to the fomorians, a new look that had more
in common with the ugly but good-hearted fairy folk
you see in folklore than with any past representation
of gnomes in D&D, and a new social role as itinerant
tinkers. In other words, it swam seriously upstream
from the cute and funny gnomes of past editions of
D&D and of World of Warcraft.
This is reaction of Bill Slavicsek regarding this vision:
“Gnomes, while still in discussion, might just
embrace the funny and cute and come up with something
playable that allows people to imagine gnomes
in ways they certainly seem to want to. We should
stop trying to swim upstream to create a cool, dark,
heroic gnome and instead turn our powers to making
a cool, funny, heroic gnome.”
And James wraps up with the following words:
That said, in putting together the first Player’s
Handbook, we wanted to assemble a collection of races
that represented the diverse population of the D&D
world and appealed to a broad cross-section of the
D&D audience. We felt, and I still think rightly, that
the gnome would have a better place in Player’s Handbook
2, where it could expand the game rather than
defining its initial parameters.
He further expands the need for gnomes for the Monster Manual and the workaround, but that is not relevant for your question.
To sum it up, gnomes were omitted from PHB1 because they were supposed to be a world expanding race rather than world defining one.