Because this is a big tournament, there is never a doubt that there will be not fewer fans and that is also one of the reasons for FIFA to increase the number of participants because it means more money will be circulating throughout the World Cup. Conditions like any of the hosts will not reduce the interest of spectators to come directly to the stadium and watch the match even though there is a possibility that the match will be boring.
I don't think that only money making was in FIFA's mind when they decided to increase the number of participants from 32 to 48. The real reason was to ensure adequate representation from all the confederations. Right now, only 9.5 slots are allotted to AFC and CAF, where almost two-third of the world's population reside. This allocation was justified back in 1990s, when the quality of Asian and African teams were not up to the mark. But now these teams are playing at a level equal to that of the UEFA teams and there is simply no justification to allot 13 slots to UEFA and 4 to AFC.
That is actually a smart thing to do. I wonder why ice hockey tournaments always have only 16 teams, that are formed by having 12 teams that got most points from previous tournaments, and 4 additional team from lower division. I wonder why dont ice hockey tournaments dont increase number of teams to 32 and have selected a method of "4 worst to best teams rotation". I can suppose that this is due to a general level of tournament, that not many teams can compete on same level with top8, even among 16 teams there are those who lose games with 4-6 score difference. Why dont football tournaments have such rotations instead of adding more countries?