They can but they won't unless they face some kind of setbacks. Remember in marketing term, SL is just a refined version of Leopard, which is partially true as in terms of front end and user features, it's not any different from Leopard, which mean for most people it's the same OSX front end since Oct 2007, not late August 2009.
I honestly think Apple split the massive OSX update into two parts, the SL release (10.6) and the upcoming huge user features pack release in order to meet their deadline of every 12-24 month OS update. For all the poking that Apple did at MS expense for the Vista's delays, they probably didn't want to face the same thing with delaying the 10.7 beyond 3-4 years. Remember that the features additions to the OS X is hard to do when the very foundations is being modernized, so they can't really have two teams working on the same OS (10.6/10.7) at same time in my opinion, so they had to split it into two OS releases.
Imagine what 10.7 would be like if they didn't, it'll probably be the fastest most impressive OS update Apple ever did at the expense of making the users wait 3-5 years for it.