That’s partially due to the diminishing returns you get with more exploded Gems. Just for fun, I just wrote a simulator that simulates explodes of 1 to 30 gems on an 8x8 board, across a million trials for each number of explosions, and calculates the average number of destroyed gems per explosion.
Exploded Destroyed
======== =========
1 7.561303
2 14.183169
3 19.996500
4 25.084244
5 29.561680
6 33.479082
7 36.946478
8 39.984386
9 42.666210
10 45.026604
11 47.112513
12 48.946650
13 50.571160
14 52.014138
15 53.282962
16 54.417065
17 55.425743
18 56.309690
19 57.102063
20 57.810127
21 58.438365
22 58.998906
23 59.499533
24 59.942867
25 60.343145
26 60.699484
27 61.016209
28 61.299726
29 61.558639
30 61.791511