They can be used in conjunction with other chemo drugs, they...

They can be used in conjunction with other chemo drugs, they just aren't typically. Medicine in general, and oncology in particular, tends to focus on one thing at a time. While somewhat a simplification, oncologists are focused on best treating the cancer (their job), not worrying about all the potential bad side effects that might happen (there are many). As a consequence they tend to worry about treating side effects when they happen - for most side effects this is fine, but in the case of cardiac damage this can't really be fixed once it happens as the heart has no regenerative capacity.

Another major aspect that makes it hard for oncologists to use pure cardioprotective drugs is the insurance companies won't pay for them (in the USA in particular) without a massive fight by the doctor. They might need to spend hours on the phone arguing with some insurance person to get prior authorisations to use the cardioprotective drug. They just don't have time to do this and treat patients. The end result is cardioprotective drugs just aren't used as often as they should.

Drugs that treat the cancer are different as the doctor doesn't need to get prior authorisation to use them. This aspect is what makes a drug like bisantrene so special - the doctor can use it as an anti-cancer drug and get the cardioprotection basically thrown in for free.