In the present scenario, wireless body area network (WBAN) use case has gone beyond medical monitoring (e.g., E-Healthcare) and has become a vital part of our daily lives. WBANs, applied in E-Healthcare systems, extract the physiological parameters through the implanted (in-body) and/or wearable (on-body) sensors applied to the human body and route them to the remote server in a systematized and well-grounded manner. Thus, routing is the most primitive and non-trivial task in WBANs for prompt and proper delivery of sensed data. The design of routing protocol is crucial for efficacious communication and high network performance. Therefore, the target of this paper is to present an all-encompassing taxonomy of the state-of-the-art routing protocols in WBANs. Firstly, this paper provides an insight into WBANs and their related projects. Next, WBAN routing challenges are explored, and after clustering these challenges based on their operating mechanism, the routing protocols are categorized into eight categories, which correspond to cluster- and tree-based, cross-layer-based, fuzzy-based, interference-based, posture-based, QoS-based, SDN-based, and temperature-based routing protocols. Furthermore, comparison tables highlighting the goals, strengths, limitations, and novelty of each protocol, along with a qualitative comparison table of the routing protocols for each category based on their features are furnished. This exhaustive survey can serve as a starting point for WBAN researchers, as open research issues have been highlighted in the discussion section.