This
And this
The issue is the scheme specific aspect of the DT you want. The usual DT is bottom heavy and designed not to move and control the LOS. Yes, there are different types of responsibilities relative to how the gap and OL are engaged and read. The A is played differently than the B. But, the 3tech has those responsibilities plus.
Now, a pass rushing 3/4 tech, is designed to not just rip through a hole, as any ILB or S can do that off the DT’s hip, but he must be able to push the OL in front of him back in order to do so. This requires an explosive first step, leverage, and a rapid drive to get the IOL back and, therefore, collapse the pocket.
As I said, in addition to the pocket collapsing, they are still responsible for the read gap responsibilities of their particular scheme. So, it is easier for a DC to find fast dudes on the edge and use an interior blitz from either the secondary or a LB than it is to find the pass rushing B gapper. Also, not every scheme uses it.
So, though pass rushing DEs/EDGE are rare, due to everyone needing them, they’re actually in a greater quantity. Now, some true A gap control 3-4 NTs or even DTs may have the ability to play a pass rushing Bgapp/3tech role, but they’re not asked to. From what I remember, the guy on the Eagles is a case in point. Actually, I think the one from Washington’s too.
In general DEs are much easier to neutralize than a 3tech, and not everyone uses a 3tech, so they’re rarer.