BTCC is glorified banger car racing. Compared to decent single seater racing, touring cars look so pedestrian. Not my cup of tea really.
How many times do we see someone just lob one up the inside and rub door handles or simply go up the back of the guy in front and tap them off the course? Can't allow any of that in an open wheel formula, unless we're happy to see people getting injured or killed.
Among the reasons touring cars is a close racing formula are; lack of aero derived downforce so no huge grip penalty for slipstreaming the car in front, ballast handicapping, enclosed wheels, no pit stops, much much lower budgets so less development options, simpler technology... Doesn't much resemble the world if F1.