The copper-wound bass strings on your piano collect dust and grime much more quickly than the plain-wire treble strings. If you don’t believe it, you can see it plainly in the picture; the portion of the string under the damper on a grand piano looks as good as new, while the exposed part of the string has turned almost brown over time. And this was not an old piano!
This is the primary cause of tone loss in the bass over time. Gunked up bass strings, unsurprisingly, do not produce good tone. The best solution, of course, is new bass strings. Measurements can be taken to replace all copper-wound strings with a complete set of new strings customized to the proper size and length (different on every piano model). New bass strings are often needed well before treble strings.
Another option is to clean the bass strings. There are several ways to do this, and a good cleaning can yield surprising improvements in tone quality, at a lower cost than string replacement. To be thorough, however, the strings have to be removed from the piano.
So there you have it. If your piano has a tubby-sounding bass, poor sustain, or just in general a dead tone in the bass, there are things that can be done! Talk to your piano technician about the options.