In those tennis games there is time to come back. Portugal were 9/4 at 1-1 with NI last night with 2 mins to go. That's a big risk for one thing to happen in a short space of time, whereas the tennis has the potential to to-and-fro and has a lot of components to each match.
Yeah the thing about football is:
-It has a timer
-Score changes happen fairly infrequently (only a few times in the whole match)
Other sports where the winner is "first to X" have the potential for big comebacks from one team/player because the opponent has to keep scoring to finish the match. Something like snooker perhaps even more so, because only one player is playing at once so you literally can't do anything if your opponent is potting everything.
Touching on the football timer thing again, I think it can help to estimate how much injury time you are expecting in a given match. People are like to lay a score change in the final minutes based on the assumption it will be a 90min (plus perhaps a standard 3mins) but if you suspect there will be say 8mins added time then it may be more attractive to back. In other words, it is kinda weird because:
-Match end time is a critical factor in the outcome of the bet
-Match end time is variable but can be predicted to some extent
(as an aside, I've noticed watching some foreign matches that referees frequently end matches too early (e.g. min 3mins added time, ref blows after 2:50), which rarely happens in England)
. That is a bit naughty though.
