  White is Player 2
score: 0 pip: 92  Unlimited Game  pip: 120 score: 0
Blue is Player 1  
XGID=aBBBBBAAccbCdb:1:1:1:64:0:0:0:0:10 
Blue to play 64 
1.  Rollout^{1}  21/11  eq: +0.2715 
 Player: Opponent:  68.72% (G:9.11% B:0.26%) 31.28% (G:11.54% B:0.26%)  Conf.: ±0.0024 (+0.2690...+0.2739)  [100.0%] Duration: 8 minutes 38 seconds 

2.  Rollout^{1}  10/6 7/1*  eq: +0.1807 (0.0908) 
 Player: Opponent:  64.76% (G:8.86% B:0.16%) 35.24% (G:9.29% B:0.15%)  Conf.: ±0.0030 (+0.1776...+0.1837)  [0.0%] Duration: 7 minutes 02 seconds 

3.  Rollout^{1}  21/15 7/3  eq: +0.1365 (0.1350) 
 Player: Opponent:  63.08% (G:7.14% B:0.19%) 36.92% (G:6.82% B:0.12%)  Conf.: ±0.0030 (+0.1335...+0.1395)  [0.0%] Duration: 7 minutes 11 seconds 

4.  Rollout^{1}  21/15 10/6  eq: +0.1139 (0.1576) 
 Player: Opponent:  62.87% (G:8.05% B:0.21%) 37.13% (G:11.18% B:0.20%)  Conf.: ±0.0027 (+0.1112...+0.1166)  [0.0%] Duration: 7 minutes 01 second 


^{1} 5184 Games rolled with Variance Reduction. Dice Seed: 7671034 Moves: 3ply, cube decisions: XG Roller

Rollout by Taper_Mike 2015Dec25 eXtreme Gammon Version: 2.10.199.2658 53P33B63S51K3143P66Z53S61O21E51p52C51J43D64OC44J51Z43PF43P41E64ntm.xgp n[S J91 c135 C158] "<=5 n[S J92 c127 C157] "&e 
eXtreme Gammon Version: 2.10
In most positions of this ilk, attacking is automatic. By hitting, you force the opponent to get lucky twice. First, he must roll an ace to enter. Second, he must roll a 6 to escape. If you leave him unmolested, he only needs to get lucky once, by rolling a 6.
So why is it a whopper to hit in this position? One explanation is the danger of cracking. Blue will face four cracking numbers (44, 22, and 42) after he hits. Four does not sound like a large number, but it represents 11% of all possibilities. Here is a variant where there are none.
  White is Player 2
score: 0 pip: 92  Unlimited Game  pip: 117 score: 0
Blue is Player 1  
XGID=aBBBBBAAcAcbBdb:1:1:1:64:0:0:0:0:10 
Blue to play 64 
1.  Rollout^{1}  10/6 7/1*  eq: +0.3576 
 Player: Opponent:  71.58% (G:10.18% B:0.16%) 28.42% (G:8.97% B:0.16%)  Conf.: ±0.0031 (+0.3545...+0.3607)  [100.0%] Duration: 6 minutes 06 seconds 

2.  Rollout^{1}  18/14 7/1*  eq: +0.3395 (0.0181) 
 Player: Opponent:  71.18% (G:10.05% B:0.18%) 28.82% (G:10.56% B:0.30%)  Conf.: ±0.0031 (+0.3364...+0.3426)  [0.0%] Duration: 6 minutes 38 seconds 

3.  Rollout^{1}  18/8  eq: +0.2276 (0.1300) 
 Player: Opponent:  66.93% (G:8.59% B:0.23%) 33.07% (G:10.16% B:0.23%)  Conf.: ±0.0022 (+0.2253...+0.2298)  [0.0%] Duration: 7 minutes 18 seconds 


^{1} 5184 Games rolled with Variance Reduction. Dice Seed: 12074957 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.10
Here is a variant where Blue will face two cracking numbers (22 and 33) if he plays 10/6 7/1*.
  White is Player 2
score: 0 pip: 89  Unlimited Game  pip: 120 score: 0
Blue is Player 1  
XGID=aBBBBBAAccbCdb:1:1:1:64:0:0:0:0:10 
Blue to play 64 
1.  Rollout^{1}  21/17 7/1*  eq: +0.3433 
 Player: Opponent:  72.04% (G:8.34% B:0.19%) 27.96% (G:11.41% B:0.25%)  Conf.: ±0.0032 (+0.3401...+0.3465)  [100.0%] Duration: 7 minutes 24 seconds 

2.  Rollout^{1}  21/11  eq: +0.2934 (0.0499) 
 Player: Opponent:  69.51% (G:9.40% B:0.29%) 30.49% (G:13.76% B:0.29%)  Conf.: ±0.0024 (+0.2909...+0.2958)  [0.0%] Duration: 8 minutes 56 seconds 

3.  Rollout^{1}  10/6 7/1*  eq: +0.2288 (0.1145) 
 Player: Opponent:  67.02% (G:7.52% B:0.16%) 32.98% (G:9.66% B:0.13%)  Conf.: ±0.0034 (+0.2254...+0.2322)  [0.0%] Duration: 6 minutes 37 seconds 


^{1} 5184 Games rolled with Variance Reduction. Dice Seed: 12074957 Moves: 3ply, cube decisions: XG Roller

eXtreme Gammon Version: 2.10
The other factor cited in this thread is that Blue will build a prime more quickly by not hitting. Is that more important than the cracking numbers? Both have an effect, but my sense is that the cracking numbers are the primary factor.