That's very different though. There's no risk factor in playing those two ahead of Lambert because they're established world class players. Bart, on the other hand, hasn't been established at all, unlike Kelvin. People are basing opinions of him off a few games four or five years ago and less than an hour of pre-season friendly. Thus it is a risk to drop a keeper who doesn't need to be dropped for the sake of just trying someone new. Cup games, yeah why not, but league games? No thanks.
Bart had a run of two months in the first team between November 2009 and January 2010 (as Kelvin was injured), he started 8 games in a row and made a very good impression.