We don't know the creation date of the HD wallet so have to sync all the blocks from the first creation date of HD wallets.
This takes a long time (minutes) and users would have that delay every time they switched accounts. This is not acceptable to users.
With one account the hit is taken once. Still not great but bearable.
(And for a newly created Trezor wallet we know the creation date as we just made it so there are a few optimisations we can make).
The sync would then only be applied on one account, so we would have to keep track of the 'last sync date' for every tip of every account.
It just gets too complicated to do reliably.
Okay, got it. You are doing a better job of syncing the wallet than the BWALLET folks on their own website. There is one tx I have where they do not recognize the change address as mine and the balance on their website is not correct (the website shows the btc sent to the change address as spent). But in Multibit HD for this tx the change address is recognized correctly and the balance is correct. So to get the correct balance I need to use Multibit HD now with the BWALLET (Trezor clone). I was considering creating another account to move the funds so the balance showed correct on their website also, but I will just continue to use Multibit HD for using this device. Thanks again.