What Mike Z said.
In CS1 when 6A could go into both JC and 4D, JC was (IMO) the better option for ending a blockstring. While 6A > 4D has value as an airtight blockstring and being +0, it doesn't give Tager any way of baiting reversals with 6A. To elaborate, 4D is +0 on both block and hit, and it magnetizes either way. The only difference in a non-CH 4D is a little bit of damage. 6A > 4D can't ever CH someone even when they're mashing, because it doesn't allow any moves to come out. What this meant for characters like Ragna/Makoto/Mu/etc. was that they could get out of every 6A > mixup attempt by mashing DP. If the Tager did 6A > 4D instead, they ate the 4D. If Tager tried to go for mixup, he ate the DP. Therefore, there is almost no reason for them NOT to mash DP, because the only consequence is a little bit of damage from a non-CH 4D. (I remember talking about all of this in another thread a while back, but I can't be bothered to find it.)
Jump cancel on 6A was the only way to keep these characters honest. Losing that meant losing mixups off 6A.