This is unfortunately a limitation for us as well 
Some banks make it basically impossible to track a transaction from pending to booked, meaning we’d loose your categorisation data as it cleared. We decided that not allowing you to categorise pending transactions was a better experience than this 
We do show them in summary however, in their own pending “category”
We’ll constantly re-assess this though, most banks are improving their data quality so if it reaches the point where we think it’s possible we will definitely re-think this