Why does Mark not make it obvious that he is referring to Jesus deity? Why do it through oblique allusions to the OT? Hays gives a number of reasons.
Firstly, the truth that Jesus is being identified as Jesus’ God is simply too radical to proclaim directly. It was simply too shocking a truth for a 1st Century Jew to say that a man could be indentified by God. So, Mark projects his story of Jesus onto the background of the story of Israel and as he does so ‘remarkable patterns emerge’ that show Jesus’ true identity.
Secondly, this elusive technique fits with Mark’s view of Jesus’ agenda. The secret of the kingdom of God is 4:12 is not given to everyone - only to those who have ears to ear. Mark’s use of the OT to reveal Jesus’ identity is only available to those who go beyond a superficial reading of the text. Mark uses his OT allusions like Jesus used the parables - to keep people out as much as to bring them in.
Thirdly, Mark 4:21-25 give us the hermeneutical key to what Mark is doing. Most English versions render v21 something like this (ESV): Is a lamp brought in to be put under a basket, or under a bed, and not on a stand? However, there are couple of interesting things in the Greek - it is not ‘a’ lamp, but ‘the’ lamp and it isn’t brought - rather it ‘comes’. Hays argues that Jesus is the lamp - and he has come to be revealed. The language in vv24-25 about paying attention to how you hear and about the measure given to you, argues Hays, and this is possibly where he strains things a bit, refer to how you read the OT. If you read the OT with a ‘generous measure’ with respect to Jesus you will see him as he really is - the God of Israel.
What Mark was doing has parallels in other Jewish writings - especially Apocalyptic. So, for example, the book of Revelation uses lots of OT symbolism to argue its point - symbolism that if you miss it can lead to nonsensical interpretations (e.g. one writer arguing that the locusts of Revelaton 9:7 referred to US Cobra attack helicopters).
Whether or not you buy all the details of Hays’ argument, I think he is convincing in showing that Mark is presenting Jesus as the God of Israel.