Because it is in the past. "just now" describes a period of time that started in the past and finished it the past, therefore you MUST use the past simple.

It is not true that "just now" must always accompany a verb in the past tense, as it does in your example. It may ALSO accompany verbs in the present, or even the future tense (or its periphrastic alternatives). Just search the British National Corpus [at http://bnc.bl.uk/saraWeb.php?qy=just+now&mysubmit=Go] for plenty of attested examples. The interesting part (= your question) is: How can that be, if "past" means 'before now' and "now" means 'now'? Shouldn´t a 'contradiction' arise? The trick is the presence of the hedge "just", which signals that the interval of time mediating between the event described and speech time is NEGLIGIBLE. Whether in absolute terms the event occurred a (negligible) bit EARLIER than speech time (as in your example) or will occur a (negligible) bit LATER than speech time does NOT matter in what concerns compatibility with "just now". (Of course, the verb will still be in the past, present, or future tense, respectively). However, interestingly, if the verb is in the past tense, "now" is possible ONLY IF "just" is also present. Remove "just" and the 'contradiction' between the time intervals that the verb "came" and the adverb "now" respectively assign to the same event makes the sentence ungrammatical: ?*"The boss came now" is bad. On the contrary, there is no problem in adding a bare "now" to a (present or) future tense, as in "I will now summarise the main points of this lecture". This is because bare "now" refers to the present, and the present is 'elastic', so to speak: it must overlap with speech time, but it may encompass a region of time that extends backwards or forwards relative to speech time (cf. "He writes children's stories"). On the contrary, the past may be vague (cf. "He wrote children's stories"), but is 'rigid': it cannot extend beyond speech time into the future (relative to speech time).

just now = а) в настоящий момент, б) только что (что случилось в данный текущий момент) We're having dinner just now. Come back later = Мы сейчас обедаем. Приходите попозже. What's your son doing just now? = А чем сейчас занимается ваш сын? @Butterfly_Leben, in your case "just now" means "a very short time ago..." = буквально сейчас секундой раньше. He left just now = Он только что ушёл. The boss came just now = Босс только что пришёл.