The opening of their official statement makes the reason quite clear [you can read the whole thing here]:

After careful review and ongoing evaluation of the regulatory landscape in the United Kingdom, we regret to inform you that BitChute will be discontinuing its video sharing service for UK residents.

The introduction of the UK Online Safety Act of 2023 has brought about significant changes in the regulatory framework governing online content and community interactions. Notably, the Act contains sweeping provisions and onerous corrective measures with respect to content moderation and enforcement.

In particular, the broad enforcement powers granted to the regulator of communication services, Ofcom, have raised concerns regarding the open-ended and unpredictable nature of regulatory compliance for our platform.