the answer is MATHS

staying as they are
2% of GBTC and 2.5% of ETHE is only at most
~$220m      and ~$100m (rounded) = 330m a year income

which wont fill the DCG $1b hole this year will it!?

if greyscale wanted to do a "buy back" of SHARES
buying back the other crap coins wont net them $1b even if they bought back 10% of multiple trusts because their holdsings are way under $1b of value

however
buying back ETHE shares is cheaper to do, to get to unlock a stash of coin out of trust.. to then sell

compared to all the other crapcoin trusts ETHE is the one that will net them the most
they then sell the unlocked ethereum coin. and then sort out their corporate holes

ETHE holdings per share is the asset value locked
market price per share is the offer they give to customer of the trust

they have 3m ether coins
lets say they want to buy back just 1mill

so they pay their trust customers the $6.16/share to but back the shares
unlocking 1m coins. which then nets the $over $1.280bill to play with

but it only cost them ~50%

they can then sell ether coins on the public exchange markets. to fill the $1b hold for just ~$500m cost

and then using the $280m spare
start a fresh ethereum coin buy up from the public exchanges at a new lower public exchange market price next month, compared to this month

and start a new ETHE trust