BCH's did NOT replace CC's. No race could build more than one a year.
BCHs were built in place of Dreads, not CCs.
From ancient research into ISC "ship classes":
Each race maintains 4 slipways, or shipyards designed to produce a specific hull. To be generic about it, they were all FF, DD, CA, DN sizes. Granted, this is not an exact science (Mirak preferred to build CLs in their DD slipway, while the Feds skipped building CLs altogether and the Klinks used their CA lines to build their D6 CLs...) but it works for purpose of this presentation.
As the General War progressed, each race noticed that the FFs were dying way too quickly, and they all wanted more CAs then ever. So, the designers went to work, beefing up the ships produced in the FF line and making a near-cruiser equivalent, thus the "war" craft were born.
The DW was designed to be built in the FF slipway, and the CW (often nearly equal to the early-era CA) was designed for the DD slipway. That left the CA slipway to build CCs, CBs and BCHs to command these new designs. And, of course, the usual variant-heaven went to work, Command, Drone, Scout, Carrier, Pac-Man, etc., variants of all the new craft were pencilled together...
The ISC ships are designed / classified to fight what the appropriate shipyard was producing
at the time of the ISC's introduction. Hence, the I-FFZ was designed to counter what was left of FFs at the time (FFB / F6 / Z-DF with appropriate fast drone upgrades actually paid for), I-DDZ vs DW, I-CLZ vs CW, I-CAZ vs CB, I-CCZ vs BCH.
If you pay attention, you'll notice that the Rommie Hawks are built on these same lines - Skyhawks are DWs, Sparrowhawks are CWs, Firehawks CAs, Nova/Regal hawks are CBs / mini BCHs, and the Killerhawk is a pocket-dread, to counter the lack of a "true" Hawk-BCH... In both cases of late-war introduction of a line, the BPV / classifications of the "new" lines fall into the appropriate ranges of everyone else's late era ships, not the early era ships.