Please explain this grammar from an ESL perspective?

You could use HAS/MANAGED if the clause stood alone, but in the context it would be confusing. Because you're talking about two time periods (the time period when he managed the Texas outfit and this morning/today) in one sentence, it's best to use a past tense (here, the past continuous) to indicate that one is finished (the time he was in Texas) and one (this morning/today) is still in progress.