Tools and weapons are not able to be put into the offhand, in order to replenish use while having mending. However shields are, because they have use in offhand.
MCPE-79164
No tools in offhand
Resolved
Duplicate
2
Unfortunately, I have a feeling that this will be closed as "works as intended." However, it is a valid complaint about the Bedrock edition of minecraft. I for one would love to see this functionality implemented as it has been on the Java edition.