It takes much less time to build a new bridge from scratch than to replace half of a bridge in-place. From the Bay Bridge FAQ (http://baybridgeinfo.org/faq):
"One of the greatest challenges in performing any seismic safety work on the Bay Bridge is that traffic must be kept moving. This has required the construction of temporary structures, and shifting of traffic from the existing bridge to the temporary structures. Major construction and demolition work is often scheduled during nights and weekends in order to minimize disruption to commute-hour traffic. This elaborate scheduling has extended the length of time that it takes to complete the seismic safety work."
"One of the greatest challenges in performing any seismic safety work on the Bay Bridge is that traffic must be kept moving. This has required the construction of temporary structures, and shifting of traffic from the existing bridge to the temporary structures. Major construction and demolition work is often scheduled during nights and weekends in order to minimize disruption to commute-hour traffic. This elaborate scheduling has extended the length of time that it takes to complete the seismic safety work."