I feel somewhat bad for leaving a -Support without reasoning as you've asked for feedback and only a handful of people responded. I was going to stonewall you with no feedback but I've decided against that. Now please enjoy my yap.
I have a litany of reasons why I left a -Support.
From my understanding, you were not a very well liked member of SA. From what I've heard you barely did anything and always complained when things didn't go your way. This isn't a very good look, even if it isn't entirely true it still reflects poorly upon you.
You also seem to hop around a lot, going from Regiment to Regiment and even from Server to Server. If you truly wish to be a USA ECM you should be attempting to hold more roles on the US server. Even though you've held roles recently, dropping them and attempting to apply for ECM really doesn't help your case.
Even though you held Site Manager, that was ages ago. You have no recent experience other than Nu-7 Captain, which as Niox said, isn't even classified as a Sr. CL4. You haven't held and ECA or an O-1 role, so you have little idea of how the inner workings of Ethics and O-1 currently operate. Getting one of these and working your way up the ranks would prove useful.
In addition, you seem largely out of touch, having ideas that would be detrimental to overall roleplay. For example, you suggested removing MC&D's ability to enter the facility without permission, I'd think as a former Nu-7 CPT you'd want something to do other than sit in watchtower and wait for raids? Even if they're inside without permission it gives you (or one of your Regiment Members) some RP. I understand that it can get somewhat repetitive but I believe it gives something for SOP to do instead of idling on surface. This makes me think you don't see the implications of what you suggest. This concerns me for what ideas you would attempt to push as an ECM.
In addition to the above paragraph, you were quite hostile to people who left negative supports. Calling people "forums alts" and seemingly not considering anyone else's opinion. Accepting criticism is a crucial part of any command role as people will be pointing fingers if a decision you make goes wrong.
Your leadership skills also seem like they're lacking. I saw you do nothing of note as Nu-7 CPT, E-11 MAJ, and Site Manager. I've been told you made good documents while Site Manager, but that is the only thing of note. Your former Enlisted and NCOs can speak more on your regimental leadership than I can. But from what I've seen while you were Nu-7 CPT, you simply complained whenever things didn't go your way and seemingly hated being a member of SOP. It isn't a very good look especially as a leader. (This is from my perspective)
I suggest you actually take lead, provide Roleplay for those under you. You are currently a Special Agent so show leadership, do patrols with Agents, give tasks for Seniors to do or perhaps escort MC&D. There is so much leadership (and roleplay) potential within DEA yet I haven't seen you take it.
My final reason is I just don't like you, I find you annoying. However this isn't constructive in any regard and isn't fair to just go off this.
In conclusion, this is largely personal opinion. However, I genuinely wanted to speak my mind so maybe you can see where I'm coming from. Obviously as a personal opinion there will be bias, and you most likely will disagree with some of the sentiment. Regardless, this is what I've seen from my perspective, although I most likely haven't seen the full picture. Nonetheless this is what I believe and why I decided to -Support. End of my yap session.