Because they actually have to PAY $ NOW for on market shares...

Because they actually have to PAY $ NOW for on market shares they buy, where as an off market bid like they are doing they get to decide what they want to do i.e. go with 50%+ if they get it or walk away if they don't with no cost to them i.e. no $ is changing hands until the 90% condition is satisfied & they get to decide if they want to vary that. They have the a lot of the game stacked to their advantage at present.

Traded vol is getting very small also, that makes getting any meaningful % on market difficult now.