This is a place we need to be careful; only a tiny part of the corpus of Greek and Roman painting has survived, consisting of vase painting, frescos from around Naples, and the Faiyum portraits plus one or two other Panels and scattered frescoes.

In general I think it's pretty misleading to talk about artistic "quality" when discussing the history of art, except as contemporary sources might have perceived that quality. Talking about the paintings of an entire civilization as "bad" is simply inappropriate.

To come at it another way, we might ask why the paintings and frescos of a previous civilization look so different from the art we consider as "good" today (or maybe in the nineteenth century - art since the early twentieth century is frequently characterized by large degrees of abstraction). This is a huge question, but some answers include that Western art today considers the system of single-point linear perspective as laid out by Filippo Brunelleschi as the proper way of representing "reality," the fact that we are used to oil paintings on a canvas support, and the ongoing influence of photography and photographic realism on our codes of perception. Another reason is, as /u/farquier and /u/Bodark43 point out, there is only limited survival of painted art from the ancient world. Even oil paintings from the nineteenth century have changed considerably since their production, due to damage, the instability of certain pigments, and other factors. These factors might presumably be much worse for art that is thousands of years old, so it is difficult to determine what original works of art looked like. Remember the fact that ancient marble statues were frequently painted, a fact that early-modern and modern Europeans tended to ignore because it did not fit their preferred aesthetic.

I agree with the comments below, by farquier and XenophonThe Athenian. The "good" and "bad" in your question have two aspects, however. One is the technology of creating a sculpture or painting, and the other is artistic technique, skillful rendering, etc. You can find, as they say, plenty of evidence of skillful rendering in the ancient world ( check out the Grotte Chauvet artwork of about 26,000 years ago, and you'll find surprising technique as well) Though the sculpture has many tricks ( not simple to make a big , tall free-standing statue balance sometimes) i's simpler- the medium is solid rock, and working it means chipping away everything unwanted. Many essential stone-carving tools stayed quite unchanged for many hundreds of years- steel chisels and wooden carving mallets. On the other hand, the technology of paintings and frescoes is more complex, and took some time to figure out, far into the Renaissance and beyond. For one thing, many important colors- lead white, cupric greens, arsenical yellows, mercuric reds- often interact badly with each other, and some have to be formulated. The simpler frescoes and encaustic ( wax) paintings of the classical era were therefore not only more fragile, less able to survive than sculptures, but limited by the materials the artists had available to work with.