Because the first session was in December 1969.
Seems like there should have been way more released on that 3 song release in December, weren't most of the Sunflower songs recorded in 1969?
That's a question that only the box set producers can answer, but I think the important date is when the tracks were
released as opposed to when they were recorded, and of course for
Sunflower that was in 1970... and working purely from memory, quite a few of the tracks that werent released on the album have been included on (relatively) recent archival releases.
Here's a list of the currently known 1969 sessions:
January
9 - session: Forever/San Miguel
13 - session: San Miguel (remake) [Sunset]
22 - session: San Miguel [Valentine - 2 sessions]
24 - session: San Miguel [Sunset]
27 - session: San Miguel [Sunset]
29 - session: San Miguel [Capitol]
February
13 - session: Got To Know The Woman (track) [Sunset]
14 - session: Got To Know The Woman (bvs) [Sunset]
21 - session: Deidrie
24 - session: What Can The Matter Be ?/Celebrate The News [Sunset]
25 - session: Celebrate The News [Sunset]
March
3 - session: Celebrate The News [Sunset]
5 - session: Loop De Loop [Western]
6 - session: Loop De Loop [Western]
8 - session: Loop De Loop
11 - session: Loop De Loop [Sunset]
12 - session: Forever (basic track) [Gold Star]
14 - session: Forever [Gold Star]
17 - session: Forever [Gold Star]
19 - session: All I Wanna Do (track) [Gold Star]
21 - session: Deidrie (track) [Gold Star]
31 - single session: Break Away [ID Sound]
April
10 - single session: Break Away vocals [ID Sound]
23 - single session: Break Away [Gold Star]
May
24 - session: What Can The Matter Be [Valentine]
July
9 - session: Slip On Through
14 - session: Slip On Through/
I'm Going Your Way (= California Slide) [Sunset]
?? - session: All I Wanna Do
August
8 - single session: Cotton Fields (45 version)
15 - single session: Cotton Fields (45 version) [Sunset]
22 - Kalinich poetry album session: America, I Know You [Wally Heider]
29 - session: Soulful Old Man Sunshine [Sunset]
September
5 - session: Soulful Old Man Sunshine [Sunset]
October
early - Add Some Music session: Slip On Through (before the 6th)
6 - Add Some Music session: Slip On Through
13 - Add Some Music session: Walkin'/Slip On Through
20 - Add Some Music session: Games Two Can Play
28 - Add Some Music session: Add Some Music To Your Day (basic track)
November
3 - Add Some Music session: 'untitled'
4 - Add Some Music session: When Girls Get Together
6 - Add Some Music session: Soulful Old Man Sunshine/Our Sweet Love/'Til I Die (demo) [Sunset]
9 - Add Some Music session: Soulful Old Man Sunshine vocals
10 - Add Some Music session: 'untitled'
11 - Add Some Music session: Raspberries, Strawberries (= At My Window) (track)
13 - Add Some Music session: This Whole World (basic track)
18 - Add Some Music session: Tears In The Morning (track)
?? - Add Some Music session: Where is She ?
December
23 - Dennis Wilson session: 'Dennis' piano interlude' (= (Wouldn't It Be Nice To) Live Again demo)
24 - Add Some Music session: Susie Cincinnati/Back Home
26 - Add Some Music session: Lady
?? - Add Some Music session:
Carnival (= Over The Waves)
?? - Add Some Music session: Add Some Music To Your Day
As you can see, the only unreleased tracks that
had to be got out in 1969 were the two I've highlighted. The rest were either already released or could wait until 2020.
