I think it's mainly because if you're only mining for 90 days, there's no ROI.
Using the stats from CoinWarz:
10 Ghash = 0.00048099 BTC/day
0.00048099 * 90 = 0.0432891
Net loss of 0.0052109 BTC - and that's assuming the difficulty stays the same for the next 3 months.
After 90 days miner is to be sold and price split between participants.
So
IF miner sold 50% of initial price there should be
0.0485
BTC/2 = 0.02425
BTC - 0.0052109
BTC ("calculated net loss") = 0.0190391
BTC profit
That is 0.0190391 / 0.0485 ~ 39% in three months

I am interested of 2 shares, but I shall have my
BTC only in couple of days.
I am using
https://bitcoinera.net/?ref=Hiljaisuus at end of month so soon as it is june I am in.
-tmu