Because he did very well at porto and seems to be a good football manager.
He came 1st in a 2 horse race with a superior squad. Then did nothing at Chelsea and cost them their CL status (assuming they dont win the final).
From Wiki:
At the start of the 2009–10 season, Villas-Boas left Mourinho's support team to pursue a career as manager, and he soon found a job in the Primeira Liga with Académica de Coimbra, filling a vacancy created by Rogério Gonçalves' resignation in October 2009. At the time of Villas-Boas' appointment, Académica were at the bottom of the league and still without wins, but their luck started to change as he introduced a new style, leading them to a safe 11th place, ten points clear of the relegation zone. In addition to that, Académica also reached the 2009–10 Portuguese League Cup semi-finals, losing against Porto at the Estádio do Dragão with a late goal from Mariano González. His impact at Académica was immediate, not only because of solid results, but also because of the attractive football displayed by the team, which led to intense media speculation linking him with the vacant jobs at Sporting Clube de Portugal and Porto in the summer of 2010.
Porto
Villas-Boas was announced as the new Porto manager on 2 June 2010. On 7 August 2010, he won his first trophy when Porto beat Benfica 2–0 in the Portuguese Supercup. He followed this with three more titles: the Portuguese Primeira Liga, the UEFA Europa League and the Portuguese Cup. Porto finished the league season undefeated, with 27 victories and 3 draws. He became the third youngest coach to win the Primeira Liga, and the youngest manager ever to win a European competition, by winning the UEFA Europa League on 18 May 2011 at the age of 33 years and 213 days. On 21 June 2011 Villas-Boas tendered his resignation as manager of Porto.
Yeah he's useless...