Car insurance, confusing?

I had a Vauxhall Astra and had fully comp insurance, I then sold the car and bought a ford mondeo the same day, I contacted my insurers and asked them to transfer the details over (i have done this previously with a different insurance company). After giving them all the details, i was told that they would not be able to insure my car, i asked why but they couldn't give me an answer. Subsequently i cancelled my insurance (so did the insurance company) and have insurance with a different company. Can anyone explain why they could not insure a ford mondeo even though i had existing insurance with them?

Only your previous insurance company can give you that answer, if in fact they are a real insurance company.

When insurance companies refuse to provide coverage, they're expected (actually required) to provide a reason for their refusal. If your insurance provider failed or refused to explain their reason, be glad you're no longer insured with them and don't go back. Because they're completely incompetent.

Some insurance companies will simply not insure certain drivers, on certain cars. Until you change your car, you'll never know if that applies to you (unless you check, BEFORE buying the new car).

Because the risk was more than they are prepared to take. I'm guessing you're young because most people would get a quote for the new car before buying it.
Young drivers are a big cost to insurers and a mondo with a bigger engine than the sluggish Astra is more likely to cause expensive losses to the insurers.

Either they will not cover that insurance group for you because of your age or record. Or the car was modified in some way. Some companies will not cover any modifications. A friend was refused because their car had aftermarket fog lights.