There are several large problems with your question, mostly in terms of how you’re defining your words. I’ll break them down before giving you some broad provisional answers.
“…less rich” is the first one to be tackled. The biggest problem here is what we mean by “rich.” Do you mean the numbers of super rich within each half (in which case the west has FAR more great aristocrats than the east)? Do you mean the median income (in which case the far urbanized and more middle class friendly east is greater)? Do you mean in productive capacity (in which case it is actually the southern Mediterranean which was more important than east or west)?
Lest you think this be an irrelevant exercise, we face the same problem RIGHT NOW when we attempt to conceptualize how “wealthy” America is compared to other countries in the world. In terms of GDP and the number of super rich individuals who reside here, we’re at the top. But in terms of median income, many other countries do better.
“…less …developed” is the second problem. What is your metric for development? Urbanization (once again, the east is bigger here in terms of number of communities, but the west is arguably larger with the presence of the mega-city Rome)? Engineering and manufacturing works (here, we could argue both halves are equal, consider evidence of the great mills in southern gaul, the pottery factories in north africa, aqueducts everywhere)?
“Always” is the other problem. The problems I’ve just laid out with respect to defining wealth and development are still present even as you try to hash out an era to compare it with. The pre-Julius Caesar Gaul was apparently dotted with pre-existing populated settlements with significant trade links to the Roman state. After conquest, they were integrated fairly quickly, and Gaul was where some of the most wealthy Roman aristocrats owned lands well into the late empire. Arguably some of these aristocrats are the ones who made the transition from Roman to Merovingian after the empire’s collapse.
I mean, I don’t want to get into too long of a ramble here because the question you’re asking is flawed to begin with, so it’s hard to address directly without cleaning up some of its underlying positions. **Basically, your presumptions are too flawed to be answerable in any meaningful way. The equivalent would be like asking, “why are asians less developed and rich compared to europeans?”** There are so many problems with that statement, that it becomes unanswerable.
But rather than leave you with an unsatisfying response of an unanswered “it’s complicated,” I’ll just say:
tl;dr – There are many ways to conceive of the western regions as NOT less developed and rich than the east. I do not outright say that the west was less developed than the east (or the opposite) because we have not agreed upon the terms of what development or rich means. However (although I haven’t expanded upon it here because it too also requires extensive elaboration), it is also clear that development and wealth changed quite a bit in the west in the time frame of the Roman Empire.