There are a two main reasons that I see, technical and business.

Generally speaking, multipoint access technologies (including cell & fixed wireless/wimax, DOCSIS, and multi-user fiber such as GPON) have a fixed amount of spectrum/timeslots to allocate for transfers. Transfer timeslots are allocated primarily for downlink (I use 75% down) because the vast majority of user traffic is download so it just makes sense to allocate that way.

On the business side it is one way to segment your customer base between business/pro users and residential users because residential users don't care and businesses are less price-sensitive. When they need more upload they are often willing to pay for dedicated symmetric connections such as TDM circuits or ethernet over fiber (the industry term is "Active Ethernet"). In theory these circuits cost more to provide because the last mile isn't shared with other users.