The trail sign was erected by the MFLNRO (the Government). If the trail was also for mountain bikes, it would require greater maintenance and upgrades. For instance, we did not remove all of the alder stocks from all of the logging roads. If it were to be mountain bike friendly, we would need to cut those to ground level on a regular basis. This is hard on our chainsaws and brush cutters.
Secondly, there are a couple soft spots on the logging roads. Heavy mountain bike use would make this a serious challenge for us to maintain and a burden on our already pressed volunteer trail crew.
Lastly, we did not design the connector trail between the lower and upper logging roads with mountain bikes in mind, and it would not survive long.
The whole valley has soft ground, dynamic creeks and a large amount of rainfall. This is part of the reason trail building in this valley has been so difficult in the past to sustain longer-term access and has challenged trail builders for decades. In essence, it has taken decades to reach this point. This is why there is the phrase "Mamquam Bush" which describes the tenacity of this rugged wildness and the mighty "red alder" that rules in dominion over it.
As a side note, we approached the mountain bike community at the start of the project and asked if they would be willing to partner with us in the construction of the new trail in order to make it dual use. They declined at the time, and so we did not have the resources to make this happen on our own.