The number of bugs is growing as is the number of packages. Some packages have many bugs because the maintainer left Debian without orphaning his packages. Some maintainers can't manage all their packages and accept to live with buggy packages.
Some bugs are sometimes too difficult to correct for the maintainer.
There's also period during which some maintainers can't work at all for Debian, but their packages may still need some attention. In general, the quality of Debian can be improved in various way. 1.2 Goal of the proposed organization Working on Quality Assurance is a big job, in fact it should not be the job of some volunteers only. Each Debian developer should take care of the quality of his packages. However, this cannot be enforced with rules, because quality is sometimes a point of view.