Why is that so ? Avignon is certainly not a big city ! But it is more than a convenient place for visiting Provence. The tourism industry say it's a 'destination'. This city offers a lot in a nutshell - restaurants, monuments, walks, museums, shopping and colorful markets -, only 2 hours and 40 minutes by TGV fast train from Paris, with sunny weather, great food and none of the big city hassles.
It is both authentic and traveller oriented. In that sense it would compare well with great cultural destinations such as Kyoto in Japan, Sevilla in Spain, Sienna in Italy, maybe Bath or Cambridge in the UK... A great alternative to a lenghty Paris stay with a standard 'Chateau de la Loire' excursion. You can actually see much more - Antique roman monuments, religious architecture, etc, you are only a stonethrow away from lovely nature spots and you get the real feel of " France profonde" (the 'real country', as opposed to Paris.
*The Euro Estate website http://www.euestate.com helps you find apartments and flatshares in major European cities (Paris, Milano, Berlin, Munchen, Wien) for both short and long term leases.
left : the Sorgue river, (which pops out of from a huge cliff at Fontaine de Vaucluse) is only 2 euros away from Avignon by public bus.