More than likely they finally got their branding consistent with the 1997 BBC rebrand.
Let's remember the BBC weren't exactly consistent which phasing out that old logo anyway. The new BBC logo first appeared on things like the BBC Sport ident and some regional news titles in 1988, but the old logo was still being used almost universially (with a handful of exceptions) throught 1989 on copyright captions. Some shows, like Eastenders (funnily enough) and Top Of The Pops were using the new logo from their first episodes of 1990 (still using the old one until the final editions of 1989), but there's plenty of shows from much later in 1990 still using the old logo on their copyrights, and even some post-1990 examples too.
Seems pretty unthinkable these days for there to be a long changeover between logos, even in 1997 the old logo vanished virtually overnight, apart from some programmes already in the can (and EastEnders kept it until the end of the year as well) and some signs on buildings which took longer to change.