I would disagree. For your first point, they've only had it up twice... not nearly enough for it to be a long-standing tradition. For the second point, this is true of ALL the game modes (including the permanent ones). Initially, I only played the standard Summoner's Rift until that got boring to me. Then I tried out Dominion and ARAM, and enjoyed the faster paced and more casual gameplay of ARAM. While ARAM may not hold the same level of attraction for me as it first did, I still find it much more fun than normal SR or TT. Most of the games I have played over the past year have been ARAM for this reason.
URF was even more enjoyable to me (I'm not saying that everyone should like it, but not everyone has to play it) as it was fast-paced and a lot more fun, in my opinion. To keep it as a permanent game mode (likely with some tweaks) would be amazing and it would most likely be the main game mode I play for some time. Sadly, judging by their seeming lack of care for the portion of LoL players asking for this I would have to agree that they are probably not going to be making it permanent anytime soon.
If I had to pin it down to a reason why they refuse the requests for adding a fun URF game mode, I would say that their focus is primarily on ranked and super competitive gaming (which is great for some, but lame for those who don't enjoy the slow competitive gameplay or those who don't give a !#$^ about the LCS), and URF has no place there with the casual unbalanced gameplay and short game length.