It turns out that Memory Alpha has a page at
http://en.memory-alpha.org/wiki/Time_travel_episodes. Using their lists:
TOS (3/5 to Earth's past):
The Naked Time
Tomorrow is Yesterday
The City on the Edge of Forever
Assignment: Earth
All Our Yesterdays
TAS (0/1 to Earth's past):
Yesteryear
Movies (2/4 to Earth's past):
Star Trek IV: The Voyage Home
Star Trek Generations
Star Trek: First Contact
Star Trek
TNG (3/12 to Earth's past):
We'll Always Have Paris
Time Squared
Yesterday's Enterprise
Captain's Holiday
A Matter of Time
Cause and Effect
Time's Arrow
Time's Arrow, Part II
Tapestry
Timescape
Firstborn
All Good Things...
DS9 (3/11 to Earth's past):
Past Tense, Part I
Past Tense, Part II
Visionary
The Visitor
Little Green Men
Accession
Trials and Tribble-ations
Children of Time
Wrongs Darker Than Death or Night
Time's Orphan
The Sound of Her Voice
VOY (2/12 to Earth's past):
Parallax
Time and Again
Eye of the Needle
Death Wish
Future's End
Future's End, Part II
Before and After
Timeless
Relativity
Fury
Shattered
Endgame
ENT (5/10 to Earth's past):
Cold Front
Shockwave
Shockwave, Part II
Future Tense
Carpenter Street
Azati Prime
E²
Zero Hour
Storm Front
Storm Front, Part II
In all, we have
18/55 time travel episodes that go to Earth's past, which is less than half. This is far from 90%, and not even most. It is less than 1/3, and actually about
32.73%. Now, there may be a few episodes here and there which are arguably in the other category, I might have erred, and the list could be incomplete, but I doubt this tally is very far off. It really seems to be a perception issue, a trend among the
prominent time travel episodes, perhaps. Or, perhaps the OP has a generic criterion in mind, yet to be stated, that excludes some of these episodes.