Well, the WNBA didn't want the team to play in the MC, while the RG was being refurbished for it's main inhabitant and biggest and most important customer (which is short sighted by the WNBA. The market in Portland is prime for the taking).
It should be noted that the game that the Fever played (vs the Wings) had less than 8000 in attendance because they played in an arena that seats about 7000 or so people (for comparisons sake, the Blazers averaged over twice that and were a shit-tactular team this season). It'd be easy for the Portland franchise, for 2 years, to play in the MC and average around the league average IMHO (last year it was about 6500 per game).
If that's the reason the WNBA didn't come, it's a dumb reason.