When people point out that Maher average 4.4 ppg in Europe it's important to note that he was bought into Besiktas later in the season not as a major piece but to bolster the second unit. While he was coming off a good NBL season where he may have been the one of the most valuable Australians in the competition, this was towards the twilight of his career and was after the tragedy that occurred with Hudson. He was playing at a high level still but his body was already showing signs of strain, his ability to contain faster guards by this point becoming an issue.
The answer to why he didn't go further is already listed many times, opportunities, partly due to the internet and partly due to scouts being open to Australians. When Heal got his chance with Minnesota following the 1996 olympics not many people believed any NBL player could move to the NBA, let alone a undersized shoot first guard who played no defence. Heal averaged 1.7ppg on 26% shooting, so he didn't exactly prove anyone incorrect there. And Heal was in front of Maher in the Boomers rotation (I would say incorrectly) so what chance did Maher have getting some interest?
In 1997 Ben Pepper, CJ Bruton and Paul Rogers got picked up as second round stash options and Anstey was drafted 17 by Dallas, the first true NBL to NBA leap (discounting Gaze with Bullets in 94 and Longley due to his college stop-over). Only Rogers sniffed the league in the second round options (but didn't play), but Anstey had a okay three year run. From there for Australians it's very sparse. Other than Gaze and Heal getting a second run both with the Spurs via Brett Brown being on staff the next Aussie in the NBA was Bogut. By this point the only Australians who have stuck in the NBA were bigs.
I would say by 2005 when guys like Penny got a shot with the change in league rules and the success of foreign guards such as Parker, Ginobili and the like opportunities were starting to open up. Penny also had a starring role on his national team, which helped with exposure.
Nowdays we have multiple guards in the league with different playing styles such as Delly, Patty and Exum and now a NBA global academy. Ferguson coming here last season helped further open up opportunities. 36ers games were crawling with scouts last season, and as a testament to how things have changed I am sitting in a bar in Khaolak right now sending links of videos to college coaches.
Now that's not to say Maher would have stuck in the NBA, but opportunities in the late 90s weren't as easy to come by as they are now.