Similarly, why is it that a good majority of goalies wear #s in the 30s?
Goes back to when numbering was first developed. In the mid-1910s in the Pacific Coast Hockey Association the players on each team wore numbers corresponding to their position. Goalies wore 1, point and coverpoint wore 2 and 3, rover wore 4, and the forwards wore 5, 6 and 7.
Gradually teams began to add substitute players to the roster, so that they wouldn't all have to play the full 60 minutes. These subs were given numbers after 7.
Typically a goalie played the whole game, and there was usually only one goalie on the roster. If the goalie got injured they would have to replace him with someone else: sometimes another player on the team, sometimes just an amateur in the audience, sometimes even the coach. (Lester Patrick, coach of the Rangers in 1928, took over as goaltender when Lorne Chabot was injured in the Stanley Cup Finals.)
After Georges Vezina of the Canadiens died Herb Gardiner and later Marty Burke, both defencemen, started wearing #1, while Vezina's replacement, George Hainsworth, wore spare numbers (10, 12). Hainsworth later wore #1 but it was the the beginning of a trend which saw replacement goaltenders wear whatever spare number was left over.
As the number of players allowed on a roster began to rise so did these spare numbers. Eventually by the '60s, when the teams would carry a backup full-time, they were well into the 20s. Roger Crozier wore 22 while backing up Terry Sawchuk in Detroit. Johnny Bower's backup, Don Simmons, wore 24. Charlie Hodge wore 25 in Montreal. Others wore the number 30 because the 20s were reserved for players on the bubble who didn't make it on the team at the start of the season but could be called up. Denis DeJordy and Gilles Villemure wore 30 behind Glenn Hall and Eddie Giacomin.
Eventually 30 became synonymous with goalies just as 1 had, especially after tandems began to take hold in the late '60s. Sawchuk wore 30 in Toronto when he was a tandem with Bower. Bruce Gamble took the number 30 after Sawchuk left. Doug Favell wore 1 in Philly, while Bernie Parent wore 30. Eventually the 'backups' wearing 30 started becoming the starter, like Gerry Cheevers did in Boston with Eddie Johnston. Many of the new starters after the 1967 expansion were former backups on the Original Six teams and they elected to keep 30, like Cesare Maniago in Minnesota.
Often if a team carried three goalies they would be numbered 1, 30 and 31 (like Chico Resch, Gerry Desjardins and Billy Smith for the Islanders), or 1, 29 and 30.
Some '70s goalies started picking numbers other than the more 'standard' 1-29-30-31, for the sake of being different. Tony Esposito picked 35 in Chicago. Gilles Meloche wore 27 in California.
After Patrick Roy started wearing 33 in Montreal (only because 30 and 35 were already taken) goalies started wearing the rest of the 30s numbers.